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THE CORRESPONDENCE OF HUGO DE VRIES AND CHARLES DARWIN

BY

PETER W. VAN DER PAS

This paper is dedicated by the author to Professor Frans VERDOORN, Utrecht¹).

In his Origin of Species, DARWIN formulated a way in which new species could originate; these views were expanded in his books The Variation of Animals and Plants under Domestication and The Descent of Man and Selection in Relation to Sex. These books described for the first time a widely acceptable mechanism of evolution. In addition, DARWIN made an attempt at explaining the then known facts of heredity in his "Pangenesis" theory.

Hugo DE VRIES is most widely remembered for rediscovering the laws of MENDEL and for his formulation of the Mutation theory, which followed soon thereafter. Especially his work on mutations strongly influenced contemporary views on the mechanism of evolution.

If DARWIN is given the title of "Father of the evolution theory", DE VRIES should be given the title of "Father of the science of genetics"; but it should be remembered that each of them contributed to the principal field of the other.

It is therefore curious that the topics about which DE VRIES and DARWIN were corresponding were not at all topics on evolution or heredity. This is explained by the fact that DARWIN, who was the senior of DE VRIES by almost 40 years, started his scientific career with his work on evolution, while DE VRIES ended his scientific career with his work on heredity.

Indeed, if Charles DARWIN had never developed his views on evolution and if DE VRIES had never worked in the field of heredity, both would still be remembered as very competent scientists, be it perhaps not as the pioneers in their field as they are now.

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DARWIN'S Origin of Species (1859) was preceded by his work on geology and his work on Cirripedia, both of which may be considered to lead to the Origin, thereafter his attention turned to the Fertilisation of Orchids (1862) and Climbing Plants (1865), Variation under Domestication (1868) and the Descent of Man (1871). After these books, DARWIN turns away from the subject of evolution, although some of his subsequent works show a casual interest in the topic. His attention turns to The Expression of Emotions in Man and Animals (1872), Insectivorous Plants (1875), Cross and Self fertilisation (1876), Forms of Flowers (1877), The Power of Movement in Plants (1880) and finally The Formation of Vegetable Mould (1881). We see here DARWIN'S scientific interests mapped out by the titles of his books, his chief means to communicate the results of his researches.

Hugo DE VRIES was born in 1848, hence the Origin was published while he was attending grammar school. While a student of botany at the University of Leiden, he read the Origin in its german translation (by H. G. BRONN, 1860) and, after due reflection, he was sufficiently impressed by DARWIN'S views to include several propositions on them in his doctor's dissertation, which is dated 6 October 1870. The topic of the dissertation itself, however, was in the field of plant physiology: The Influence of Temperature on the Vital Phenomena in Plants²).

Of the propositions in the dissertation of DE VRIES, one has a direct bearing on DARWIN'S views, while a second one suggests familiarity with one of DARWIN's books. The first of these is:

VIII, The hypothesis of Pangenesis (DARWIN, Animals and Plants under Domestication) cannot explain the variability of species.

This proposition shows that DE VRIES had studied the book by DARWIN to which he refers and that he had dissenting views at this time.

The other proposition:

IX. The fact that crossfertilisation of plants in some cases has a better result than selffertilisation is only a result of the fact that

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selffertilisation is avoided by the shape of the flower, but not its cause.

At a later date (1876), DARWIN had much to say on this topic, but his book of 1862, of which the full title is: On the various Contrivances by which British and Foreign Orchids are fertilised by insects and of the good effects of intercrossing, may have inspired Hugo DE VRIES to write down this proposition. It must however be remarked that in this book, DARWIN says little on the good effects of intercrossing which he promises in the title.

We may safely assume that, at the time of his promotion, Hugo DE VRIES was thoroughly familiar with all the works of Charles DARWIN which had been published at that time and that he kept up with DARWIN'S publications of later years.

The winter semester which followed his promotion, DE VRIES spent in Heidelberg as a student of HOFMEISTER. HOFMEISTER was originally an amateur botanist, whose botanical work was of sufficient importance to obtain him the chair of botany in Heidelberg at the age of 39 years. Part of his activities were in the field of plant physiology, such as the study of the curvature of growing parts of plants, to which subject DE VRIES would soon start contributing. During the semester of 1870–71, HOFMEISTER was already in ill health and it seems unlikely that Hugo DE VRIES did much fruitful work in HOFMEISTER'S laboratory. There is no paper which reports results from this period.

The summer of 1871, DE VRIES spent in Würzburg, working in the laboratory of Julius SACHS. In September 1871, he was appointed botany teacher at the secondary school (HBS) in Amsterdam; a position which he held until April 1875. Almost every year, DE VRIES spent his summer vacations with SACHS in Würzburg, until he finally obtained a permanent position there; he was charged with the preparation of a series of monographs on agricultural plants by the Prussian Ministery of Agriculture.

The three summers which were spent in the laboratory of Julius SACHS were entirely devoted to research on the cause of movements in plants; they were reported primarily in the Arbeiten des Botanischen Instituts zu Würzburg, the journal of Julius SACHS, although this work yielded papers in some other journals also. The Würzburg papers will be briefly discussed here.

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For the summer of 1871, SACHS had given Hugo DE VRIES the task to experimentally examine the difference in geotropical and heliotropical movements of two sided symmetrical parts of plants, as compared to vertically growing systems³). In his experiments, DE VRIES eliminated the influence of gravity (geotropy) and light (heliotropy) and so doing found a third agent which influences the curvature of plant parts; a natural tendency of one side of a plant organ to grow faster than the other side. If this growth was fastest at the ventral side, DE VRIES called it epinasty; if the reverse was true, hyponasty. The experimental work of DE VRIES showed that stems and ribs of leaves were 1) negatively geotropic and 2) epinastic. The epinasty decreased with the age of the organ; in some cases, very young organs were hyponastic. Heliotropy was absent in most cases and, if present, considerably smaller that the hyponasty. For unmutilated, nonvertical shoots, the results were identical. The three agents: geotropy, heliotropy and epi- or hyponasty could, according to DE VRIES be chosen in such a way that the growth of the plant in any direction could be explained.

In the same year, SACHS asked DE VRIES to find the cause of the wilting of cut-off sprouts. SACHS himself had already made some experiments on this subject; the work of Hugo DE VRIES⁴) did not actually elucidate the question. He ended his paper with the remark: "A more precise knowledge of how the decrease in conductivity depends upon the various circumstances and the discovery of the true cause must be reserved for further experiments."⁵)

In the summer of 1872, DE VRIES studied the behavior of free moving tendrils and of tendrils which have been exited by an obstacle. This subject had been studied before by Hugo VON MOHL⁶) and Charles DARWIN⁷) in 1827 and 1865 respectively. The results of these experiments were summarized by DE VRIES⁸) as follows:

1) The influence on the difference in growth which is caused by excitation of the upper and lower side of tendrils is not only local, but extends from the excited spot for a shorter or longer distance; sometimes even along the entire length of the tendril.

2) The influence, caused by excitation on the difference in growth of the upper and lower side does not stop immediately after the contact has ceased, but lasts for some time after the exciting object has been removed.

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3) The magnitude of the difference in growth, caused by the excitation does not depend on the thickness of the exciting object, but rather on inner causes. Normally, the tendril attempts to curve more than called for by the thickness of the support, and in this way presses itself firmly against it. Experimental work on the growth of a curving tendril showed that the upper side of the uncurved tendril always grows considerably stronger than the upper side of the uncurved tendril and that the lower side of the curved tendril grows considerably less, not at all, or even contracts.

In this paper, Hugo DE VRIES refers to the offprint of DARWIN'S paper, but this does not necessarily mean DE VRIES was in contact with DARWIN at his time; the offprint was probably sent to SACHS.

The work in Würzburg during the summer of 1872 yielded a second paper⁹), on the mechanical causes of movements of twining plants. This is a veritable masterpiece. Here DE VRIES took up the question of whether the curving around a support by a shoot of a twining plant is initiated by irritating the plant organ which is concerned, as was claimed by VON MOHL, or whether this happens without the necessity of excitation as maintained by DARWIN and some others before him. The shoots of these plants describe a sweeping motion (nutation) and DE VRIES showed that the curvature of the shoots around an object is not caused by prohibiting the nutation. His experiments showed, among others, that twining plants can only wind around vertical or nearly vertical objects. His conclusion reads as follows:

Twining plants are not excitable. Any side of the shoot can become the concave one and, since in the growing part of the shoot a torsion is usually present, any side of the shoot becomes the concave side in turn and touches the support. The helical windings are caused by the inhibition of the rotating nutation. The helical windings stretch themselves as they grow and this way press themselves against the support which they surround; if there is no support, the shoot becomes straight. In twining stems, the torsions which are caused by internal agents combine with torsions which are caused by outside agents and which wind either in the same or in the opposite direction.

Janus, LVII

12

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The investigations of Hugo DE VRIES in Würzburg¹⁰) for the year 1873 are not of importance for the study of his relations with DARWIN, it will be sufficient to record that this investigation was a preliminary study into the mechanism of growth of plant cells which consisted of measuring the rate of elongation, stiffness and torsion in growing, turgescent stems of plants and that the result was that these properties increased towards the younger part of the stem.

During the summer of the year 1874, Hugo DE VRIES spent only a few days in Würzburg. That year, he was a member of the committee which supervised the final examinations of the secondary schools; this activity required extensive travelling in Holland and left no time for an extended stay in Würzburg.

Charles DARWIN'S paper on climbing plants was published in book form¹¹) ten years later (1875) "in a corrected and, I hope clearer form, with some additional facts". Since its first appearance, in 1865, DARWIN had learnt of the investigations of DE VRIES¹²). It is unlikely that Hugo DE VRIES sent DARWIN preprints of his papers at this time, but it is possible that Julius SACHS had done so, or that DARWIN subscribed to the journal.

In the preface of his book on climbing plants, DARWIN praises the work of DE VRIES in no uncertain terms:

Recently two important memoirs, chiefly on the difference in growth between the upper and lower sides of tendrils, and on the mechanism of twining plants, by Dr. Hugo de Vries, have appeared in the "Arbeiten des Botanischen Instituts in Würtzburg", Heft 3, 1873. These memoirs ought to be carefully studied by everyone interested in the subject, as I can here give only the references to the most important points. This excellent observer, as well as Professor Sachs, attributes all the movements of tendrils to rapid growth along one side; but, from reasons assigned towards the close of the fourth chapter, I cannot persuade myself that this holds good with respect to those, due to a touch.

In the text, DARWIN refers to DE VRIES five times in such terms as: This whole subject has been ably discussed and explained by ……(p. 9), or: Dr. de Vries also has shown by a better method than that

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employed by me, that the stems of twining plants are not irritable, and that the cause of their winding up a support is exactly what I have described (p. 16), etc.

He even acknowledges to having his knuckles rapped by DE VRIES for overlooking a sentence in VON MOHL'S treatise (p. 165), but he adds: But I am not surprised that this brief sentence, without any further explanation, did not attract my attention.

DARWIN sent a complementary copy of his book to DE VRIES. This book is now in the library of the Hortus Botanicus in Amsterdam. It bears the dedication: To Dr. de vries, with the respects of the author. Hugo DE VRIES acknowledged this courtesy with the following letter¹³):

Würzburg 7 Nov 75

Dear Sir:

Please accept my sincere thanks for your great kindness of presenting me with the second edition of your work on Climbing Plants. I thank you even more for the kind recognition of my two papers on the subject, which should not be entitled to such honor as you bestow on them.

I found your valuable present only a few days ago, when I returned to Würzburg from quite a long trip; and now I have the pleasure to study it closely. My cordial thanks for the new knowledge which I find here, as always in your works.

Please allow me to use the occasion to express my admiration for your last work, about carnivorous plants. This summer I had an opportunity to repeat all main experiments, partly in the laboratory of Hofrat SACHS, partly on my own, and was able to convince myself of the correctness of your observations and conclusions, and learned to appreciate and admire them.

You object to the idea that the difference in longitudinal growth at both sides of tendrils is the cause of their curvature. I must confess that you have touched a weak spot here. My experiments prove essentially only that the curvatures are accompanied by such a change in longitudinal growth; and I confess that it is a purely theoretical idea, not proven for specific cases, that I have postulated this change in growth as a cause of the curvatures. Your idea, that

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the curvatures have another cause and themselves cause the difference in growth rate, seems to me as valid as the other one, and the facts which you introduce speak very much in your favor.

Your objection concerns, as I believe, all so called growth curvatures in equal fashion and I would therefore not give a verdict until the question has been studied from a more general point of view.

Thanking you again for your kind inclination towards me, I sincerely recommend myself to you and sign

With true regard
Your obedient servant
Hugo de vries.

It must be remarked that, although the book which Hugo DE VRIES received from DARWIN carried the imprint: second edition, it was actually the first edition in book form; the paper of 1865 being considered the first edition.

The experiments by which Hugo DE VRIES confirmed DARWIN'S experiments on carnivorous plants were made because Charles DARWIN had sent a complementary copy of his Insectivorous Plants (1875) to Julius SACHS, who had corresponded with him on the subject before. In a letter to Hugo THIEL¹⁴), dated the first of August 1875, SACHS wrote:

About five weeks ago, Darwin sent me his latest work on insectivorous plants, even before it was available in the book stores. I found so much remarkable information therein that I immediately ordered Drosera from München, Dresden, Darmstadt, Holland, Jütland Kiel and so on and bought Dionea, Nepenthes, Cephalotus; de Vries supplied me with Utricularia and Pinguicula; and thus I was able to repeat the most important experiments, and yesterday, I could give a lecture with demonstrations before the Physical Society, which attracted many people¹⁵).

The explanation in the letter of Hugo DE VRIES refers to the critical remark in the preface of DARWIN'S book, already quoted above and to a passage on pages 180–181, which reads as follows:

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When the extreme tip of the tendril of Echinocystis caught hold of a smooth stick, it coiled itself in a few hours twice or thrice round the stick, apparently by an undulatory movement. At first I attributed this movement to the growth of the outside; black marks were therefore made, and the interspaces measured, but I could not thus detect any increase in length. Hence it seems probable in this case and in others, that the curvature of the tendril from a touch depends on the contraction of the cells on the concave side.

and, after a quotation from the botanical textbook of SACHS:

It must not however be supposed from the foregoing remarks that I entertain doubt, after reading de Vries' observations, about the outer and stretched surfaces of attached tendrils afterwards increasing in length by growth. Such increase seems to me quite compatible with the first movement being independent of growth.

As is seen in the letter which was quoted above, Hugo DE VRIES admitted that he might have generalized too much and stated that a more general law could only be formulated after more experimental work had been done. It was however only several years later that DE VRIES returned to the subject.

Unfortunately, only one of the letters of Charles DARWIN to Hugo DE VRIES has been preserved and this only because this letter was probably among the letters, preserved by his wife. Hugo DE VRIES apparently did not preserve his letters during this period. We can therefore only guess at the contents of the letter by which DARWIN answered, which must have been an encouragement to continue his experiments with two species, with which DARWIN had experimented himself.

The reply of Hugo DE VRIES¹⁶) has been preserved:

Würzburg 17 Nov 75

Dear Sir,

Please accept my cordial thanks for your last letter. I do not know whether I will have an opportuninty to continue my experiments with tendrils next summer… if this is the case, I will consider

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it my first task to observe the facts which you mention in your work.

I hope that I will have managed to obtain Echinocystis lobata and Passiflora gracilis by that time; however I think that tendrils of other, very irritable plants, will show the same (phenomena). It will then probably be necessary to interpret the relations of longitudinal growth for all curvatures of tendrils in another way than I have before.

Recommending myself to you

With great respect
Your obedient servant
Hugo de Vries

It will be noted that these letters were written from Würzburg, where Hugo DE VRIES had, at this time, obtained a commission to write some monographs on agricultural plants. By this time he had decided to follow an academic career and Julius SACHS obtained for him the position of privat dozent at the university of Halle. According to prussian law, only those in the possession of a doctorate which had been granted by a german university were allowed to teach in german universities; therefore DE VRIES had to write a Habilitationsschrift¹⁷), the topic of which was, to a certain extent, a continuation of the work, done in the summer of 1873 and which topic foreshadows his later work on the plasmolysis of cells. The dissertation was defended on the twelfth of February 1877, while his inaugural lecture¹⁸) was presented on the fourteenth of the same month.

The experiences of DE VRIES at Halle were not happy ones and he wondered whether an academic career in Germany should remain his goal. Events in Amsterdam came to his rescue.

The city of Amsterdam had been conducting a school of higher education since 1632. This school, the Athenaeum, was almost of university status; it did however not grant degrees. In order to obtain degrees and doctorates, the students had to go to the recognized universities, such as Leiden or Utrecht. Several of the ancestors of Hugo DE VRIES obtained their education at the Athenaeum. For a long time, the city council had desired to raise the status of the

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Athenaeum to that of a full university; finally (1877) a law which recognized the school in Amsterdam as an university was passed in the Hague. Consequently, the school in Amsterdam had to offer all courses which were customary at universities in those days; this created a number of teaching positions to be filled. The position of lecturer of botany was offered to Hugo DE VRIES in September 1877. Already the next year he was promoted to extraordinary professor, which office he started with an inaugural lecture on photosynthesis¹⁹), delivered on the fifteenth of October 1878.

In the summer vacation which preceded his first year as a professor in Amsterdam, Hugo DE VRIES made a last trip to Würzburg, where he met Francis DARWIN, the botanist son of Charles DARWIN, and subsequently a trip to England, to meet the leading botanists there. On this occasion, he met Charles DARWIN personally for the first and only time. We are fortunate that an account of this visit, as described by Hugo DE VRIES himself, has been preserved in three letters, which follow here²⁰).

The first of these is addressed to his maternal grandmother, Mrs. Louise Sophie REUVENS-BLUSSɲ¹):

Buecker's Hotel, London
Christopher street
9 August 1878

Dear grandmother,

Today, I wrote to Darwin to ask him when it would be convenient for him to receive a visit by me. I sent the letter to his address in Down; his son Francis had told me that he would be there.

Yesterday, I received an answer of Darwin from which I learned that Francis had given me the wrong information. Darwin is not at home but at Dorking, where he is staying for some weeks. I am sorry about this, for I had not only hoped to talk with Darwin but also to see the place where he has done so much for science. Now I have written to Dorking to find out whether it is convenient for him to see me there.

To get acquainted with Hooker and Thiselton-Dyer, the directors of the botanical gardens ar Kew, was for me equally impor-

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tant as visiting Darwins. Therefore, 1 wrote to T-D; he is the son in law of Hooker, the famous botanist, whose name must be familiar to you, were it only because he was the first one to draw the attention of the entire civilized world to the mode of life of insect-eating plants, about one year before Darwin's book on the subject was published. T-D answered very politely and invited me for dinner today, Friday evening; he wrote me that I would meet Hooker at his house. This was more than I dared hope for; I had visualised that he would simply introduce me to Hooker.

I went to Kew at about 3 o'clock and visited Mr. Dyer to tell him that I was pleased to accept his invitaion. He said that he was sorry not to be able to conduct me personally through the garden, but he gave me a guide to the hothouses, with the help of which I inspected the hothouses and the gardens from 3 to 7 o'clock.

About seven, I went to his house, a very charming villa near the railroad station. There I met, in addition to his wife and Sir Joseph Dalton Hooker, an aunt of Dyer, who was a guest and a certain Mr. Armstrong from London. I had the honor to lead Mrs. Dyer to the dining room and to be seated between the two ladies. Some time in the future, I will give you a verbal report of the dinner.

Of course, I was again believed to be a son of Professor DE Vriese. Hooker told me much about his visits in Holland many years ago and of the botanists of those days. Further about insect-eating plants and many other subjects. After dinner we were served coffee and tea and I left at 10 o'clock with the North London Railway.

It was fortunate that I went today, because Hooker will go on a trip tomorrow and it was very kind of Mr. Dyer to ask me over today, since he actually did not have the time to entertain me today. He asked me to return to Kew tomorrow to see the new buildings for the herbarium and his laboratory, where I will make my acquaintance with Burdon Sanderson, who is also a well known plant physiologist. Therefore, I will be going back tomorrow.

It is late now, almost 12 o'clock, therefore I will end here and mail this letter tomorrow.

Kind regards of your grandson
Hugo de Vries.

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Willem Hendrik DE VRIESE (1807–1862) was a professor of botany at the university of Leiden; he was no relation to Hugo DE VRIES; the family name is different.

The house in Dorking, where Charles DARWIN was staying, and where Hugo DE VRIES visited him a few days later, was Abinger Hall; it belonged to Sir Thomas H. FARRER. FARRER was a brother in law of Charles DARWIN; he married Ephemia WEDGWOOD, who was the sister of DARWIN'S wife Emma, in 1873. ²²). FARRER was associated for a long time with the Board of Trade and served there as assistant secretary from 1854–1865 and as secretary from 1865–1886. But his biography ²³) does not mention that he also was a very able botanist!

Dorking is situated in the county of Surrey, south of London. The little map identifies the geographical names, mentioned in the following letters.

HOOKER and THISELTON-DYER were introduced in the letter of Hugo DE VRIES; Mr. ARMSTRONG could not be identified, Sir John SCOTT BURDON SANDERSON²⁴) was a physician who was famous as a physiologist, mainly human and animal, but interested in plant physiology also.

In the month of July 1878, Hugo DE VRIES had become engaged to miss Elisabeth Louise EGELING, the daughter of a well known physi-

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cian of Haarlem (he married her on 10 April 1879) and to her he addressed the letter which described his visit to DARWIN²⁵):

London, 14 August 1878

Today I have visited Darwin; I am happy that it happened and I must say that Darwin was so very cordial and friendly that I appreciate having visited him. I was there from 2–4 o'clock; at 3 o'clock he had to rest for one half-hour to avoid becoming too tired; during this time I took a stroll through the grounds with his nephew, Mr. Farrer.

At first, I was received in the family, Mrs. Darwin, her daughter and some ladies Farrer, later I was alone with Darwin in his room and afterwards took a stroll with him until 3 o'clock. At about quarter to four, I had coffee with Darwin and Mrs. Farrer and subsequently was brought to the train in their carriage.

The conversation was quite easy; they all spoke very slowly and clearly and they gave me the time to speak up; thus I did better in speaking english than I expected I would and, in spite of my shortcomings, the ladies felt they had to complement me for my good english, even if it was unjustified. In the garden there were hothouses with peaches and grapes. Darwin told me a long story about the peaches and immediately offered me one of them; it was delicious!

During our scientific conversation, there was the same laughing mania as you have seen with Sachs; Sachs laughs all the time, Darwin somewhat less but as merrily. He was very interested in what I have done lately; it was fortunate that these were subjects which he had wanted to study himself, but he did not get around to it. He told about his own research; for example, when he had just discovered the remarkable contrivances in Orchid flowers, long before he published about this, Huxley came to visit him. He told everything to Huxley, who answered; And my good fellow, would you have me to believe all this? This is long ago and now everybody knows that it is the truth. He told me he had trouble learning languages and that his wife and sons helped him in reading german books. Why was it that they could never decipher Pfeffer's books? Was the german so difficult? my opinion was that I usually did not

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Julius Sachs with his students Hugo de Vries, Josef Schuch and Josef von Baranetzky. Würzburg, August 1871.

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Charles Darwin. From Vanity Fair, Men of the Day, No. 33, 30 September 1871.

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understand him myself and that the trouble was not in the german language, but in lack of clarity in the style. This pleased Darwin very much.

Had I ever done work on the pollination of flowers by insects? What is the purpose of the "wigs" in the "wig tree"? That there are so few plant physiologists in England. That it is regrettable that all of us do not write in english. He likes Sachs very much and we talked a lot about him. He puts a footstool on a chair before he sits down on it, for he gets headaches if he sits low—the poor soul! Mr. Farrer told me that today he (Darwin) felt exceptionally well and happy and that I was lucky. Mrs. Darwin takes good care of him and will never allow him to become too tired; she simply sends him to bed! One time Hooker told Darwin that, without such a wife and such a quiet life he never could have done so much for science as he actually did.

Darwin was very kind and said that he would have written me to come over to Dorking, but that he was afraid that this would be too much trouble for me for a mere visit to him. This was a little too polite, don't you agree? Next Friday, Darwin leaves for a place which is even farther away, therefore I was lucky that I could see him now. He has deep set eyes and in addition very protruding eyebrows, much more that one would say from his portrait. He is tall and thin and has thin hands, he walks slowly and uses a cane and has to stop from time to time. He is very much afraid of drafts and generally has to be very careful with his health. His speech is very lively, merry and cordial, not too quick and very clear.

It is remarkable how soon one feels at home with people who are friendly and cordial. What a difference with Hooker and Dyer; they were cold and I did not care about them. But I enjoyed my visit with Darwin and I feel much more happy these last days. It is such a pleasure to find that somebody is really interested in you and that he cares about what you have discovered…

The delightful description of DARWIN, given by DE VRIES in this letter reminds vividly of the DARWIN portrait in Vanity Fair.

Wilhelm PFEFFER was a german plant physiologist, who was universally disliked; at a later time DE VRIES said of him that he had illusions of grandeur.

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The wig tree (translated from the dutch pruikeboom) is the Rhus Cotinus, which is called the smoke tree in the United States. According to Asa GRAY²⁶) …most of the folwers are usually abortive, while their pedicels lenghten, branch and bear long plumy hairs, making large and light, feathery or cloud like bunches, either greenish or tinged with red, which are very ornamental.

Hugo DE VRIES reported on his visit with DARWIN to his grandmother, Mrs. L. S. REUVENS BLUSSÉ in a somewhat different way. In this letter he says a little more about his scientific conversation with DARWIN ²⁷):

Christopher street
Bueker's Hotel
14 August 1878

Dear Grandmother,

Today the actual goal of the trip has been reached and I hurry to write you about it. I have talked with Darwin. I have visited him and was received so kindly and cordially as I never had dared hope for. But let me start at the beginning. He had written me about which trains I could take to come and leave and left me the choice of the day. I answered that I would come today.

I left by train at 11 o'clock and arrived at Gomshall at 1 o'clock, I had lunch in the village and afterwards rented a carriage which brought me to Abinger Hall, the country house of Mr. Farrer, with whom Darwin was staying. Mr. Farrer seems to be a brother in law of Darwin; his son called Darwin: uncle Charles.

I handed in my card for Darwin, was ushered in and received very cordially. He proposed to introduce me to the family first and have a private talk in his room later. I was introduced to Mrs. Darwin and her daughter, and to Mrs. Farrer and her son and daughter; Mr. Farrer was in London on business. We talked for a short time about all kinds of things, the country house (which is very large and beautiful), the surroundings (also very beautiful), politics, my journey etc. Thereafter Darwin took me to his room and we talked about scientific subjects. At first about tendrils, in connection with our former correspondence. He gave me arguments for the opinions he had voiced and asked if I had done further work

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on the subject. I already had designed a method to solve his difficulties, but had not been able to obtain the plant which he had recommended, either in Würzburg or in Amsterdam; the head gardener (hortulanus) could not find the plant either. Darwin had obtained his seeds from Asa Gray in America and promised to ask this scientist for new seeds, to give to me.

He asked me what I had been doing lately and it so happened that I was working on a subject which interests him very much: the withdrawal of plants into the ground in winter, to protect themselves from frost. This is caused by a shortening of the roots, which I have observed and measured. It happens in richly foliated annual plants. Darwin had read a remark somewhere that a similar phenomenon is observed in seedlings; this spring he had sown the species involved to observe for himself, but the seed had not come up. He was very happy to hear from me that large plants do the same thing also, that this happens quite generally and appears to have a specific purpose in the life of the plant: protection in winter. We talked about this for a long time. Further we talked about Sachs, Pfeffer, all kinds of literature and about his son Francis' staying in Würzburg, too much to write down.

Afterwards he proposed to take a walk in the garden; he pointed out all kinds of plants about which something could be said and told me many interesting things. He became tired and had to take a rest; he is not allowed to talk for a long time and especially not so intensively as today. However, he proposed that I should stay and have coffee with him and the family after about half an hour. In the mean time, son Farrer would walk with me outside and show me all kinds of things. This happened.

During the coffee hour, Darwin was apparently still tired, but happy; he made me tell all kinds of things about Holland and told me himself about his travels around the world. When it was time to depart, I took leave and was brought to the railway station in the carriage of Mr. Farrer. I had known all the time that Darwin is very kind and cordial, but I had not anticipated such kindness and cordiality as he bestowed on me today. Mr. Farrer told me that today, Darwin felt exceptionally well, but yet it was obvious that he is weak and ailing. He is a tall man and his portrait is a good likeness, although from the portrait I thought he was small. Later

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I will tell about all this in more detail; it is not possible to write all of it down.

Your loving
Hugo DE VRIES.

P.S. Last Monday, Professor Suringar was here; I visited Burdon-Sanderson with him at Kew. He very kindly showed us his laboratory and his apparatus and demonstrated it.

Already the next day DARWIN fulfilled his promise of writing to Asa Gray ²⁸), asking him to send seeds to DE VRIES:

Down, Beckenham, Kent.
Railway station
Orpington S.E.R.
Aug. 15-th 78

My dear Gray,

Dr. Hugo de Vries, who has done such excellent work on climbing plants is, at my suggestion, wanting to make some observations on the tendrils of Echinocystis lobata (I think this is the right name, but I am writing away from home) of which you formerly sent me seeds. He has tried to get seeds and I have offered to write to you. If you can, I beg you to send him seeds, addressed to:

Prof. Hugo de Vries,
Amsterdam

He is professor of botany there.

I see that we both are elected corr(responding) member of the Institute. It is rather a good joke that I have been elected in the botanical section, as the extent of my knowledge is little more than a Daisy is a compositous plant and a Pea a leguminous one.

Ever yours very sincerely
Ch. Darwin.

I am very anxious for de Vries to test many of my observations on Echinocystis.

The conversation on the contraction of roots had evidently interested DARWIN very much; the next day already he made observations during

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his daily walk and reported to DE VRIES in the following letter ²⁹):

Abinger Hall
(Gomshall)
Aug. 16-th (1878)

My dear Sir,

How easy is it to see a thing when it once has been pointed out! I pulled up some wild parsnips and the upper part of the primary root was finelly corrugated with transverse folds. Then one sort grew on (the) edge of Leith cliff and the upper part must have been kept very dry, and it was much more corrugated than the roots of other plants. I dare say, you have thought of expiring plants, so the upper part of the root might be kept very dry or damp. I will mention one other point: farmers say the growing wheat (Triticum) plants are easily ejected by frosts out of the ground, by the ground being trampled or rolled. Therefore I pulled up some plants of wheat and with a pocket lens could see no corrugation.

Can this have any connection with the abortion of the primary root in the Gramineae?

Forgive me for switching this and amusing myself.

I much enjoyed seeing you yesterday and remain

yours very faithfully
Ch. Darwin.

P.S. I have written to Asa Gray.

Hugo DE VRIES immediately replied:

Buecker's Hotel.
G. Hohly, proprietor
1, 2, 3, 4, Christopher street
Tinsbury square, London.
Aug. 17, 1878.

My dear Sir!

I am very much pleased that you have thought it worth while to control (i.e. check) my little communication, and to examine yourself some roots of plants to see their folds. The root you sent me

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shows the phenomenon very clearly. I have not yet experienced (i.e. experimented) on the influence of dryness or humidity, but I have found that cut roots contract also (in 24 hours) if lying wholly in water. However, I observed with Carum Carvi that roots of plants, growing in fissures of the earth showed more folds than those of plants of the same field, that had grown in compact earth.

Should it not be possible that specimens, growing in loose earth or in fissures contract more because they are not hindered in doing so, whilst plants in compact soil have to surmount a great resistance, and become therefore less shorter, than they would do otherwise?

I am much obliged for your notice on Triticum roots, I have not yet paid attention to them and am therefore not able to answer your questions, but I will examine it directly (i.e. immediately) after my returning to Amsterdam.

I am also much obliged for the kindness you had in writing for me to Prof. Asa Gray, and beg you to accept my sincere thanks for the benevolence you showed me in receiving me so very kindly at Abinger Hall.

I remain

my dear Sir
Yours very truly
Hugo de Vries.

At a much later date, in 1925, Hugo DE VRIES remembered his visit to DARWIN as follows in an interview with Mrs. VAN ITALIE-VAN EMBDEN ³⁰):

"I was led to the study of heredity by … my love for Darwin".

"Have you known him?"

A light went up in his eyes. "I visited him. He was old and I a fledgling. He was lying on a couch and I was allowed to sit next to him. At that time, Darwin was the only one who produced in this field. Now it is so vast!"

The experimental work on the contraction of roots was performed by DE VRIES in the years 1878 and 1879. A preliminary account ³¹) was published by the Royal Academy of Science in Amsterdam, which

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had elected DE VRIES a member in 1878. The definitive report appeared in the Landwirtschaftliche Jahrbücher³²), to which DE VRIES still contributed. This work may be considered to be a continuation of the work, done in Würzburg in 1873.

HUGO DE VRIES actually received the seeds which DARWIN had asked Asa GRAY to send him; and in the summer of 1879 he had an opportunity to use them. He reported about this work in the following letter to Charles DARWIN:

Amsterdam, Kerklaan 9
Aug. 7 1879

Dear Sir

During the last days I have been making the experiments on tendrils, you had the kindness to point out in your letter of Nov. 10, 1875. I am much pleased, that I have at last found occasion of making these experiments; you know that it was chiefly with regard to them, that I have worked out in 1876 my plasmolytic method in my paper: Untersuchungen über die Zellstreckung 1877. This method has now proved of great use to me.

Among the seeds of Echinocystis lobata, only one germinated and gave a small plant; the seeds of Sicyos lobata, that Prof. Asa Gray sent me at the same time, germinated very well, so I have made most of my experiments with the tendrils of this species.

The question was to decide, whether the rapid curvations of the tendrils are caused by growth or by a change of the turgor in the cells. So I put the tendrils, as soon as they had curled clearly around the thin (2 mm) sticks, in a solution of NaCl of 200%, where the turgor was annulled in a very short time.

Tendrils that had made ¼-½ curvations in ¼-1 hour, quite lost them in the salt solution, and showed thereby that your suggestion was right, and that no appreciable growth had occurred on the upper side. Tendrils that had curled once or twice round the sticks, did not quite loose their curvations, but lost them the more, the less they had curved themselves. So it was also with the tendrils of other plants.

You see that the stimulus occasioned a change of the turgor of the cells, and that the growth is increased only in a secondary manner.

JANUS, LVII 13

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It seems that by all curvations of growing plants, the turgor of the convex side is increased first, and that the increasing of the growth is only an effect of the increasing of turgor. For they all loose their curvations more or less in the salt solution. So it is with the epinastical curvations of tendrils and of petioles, with the revolving and climbing movement of climbing plants, with the geotropical and heliotropical curvations of young stems and with the geotropical curvations of the knots of grasses.

If you cut off tendrils, that have just curled themselves round a stick, or made some free curvations after not finding a stick, and you put them in a solution of salt of 20%, you will easily see that the number of curvations becomes smaller.

I am yet extending my investigations on this point.

I have also experienced (i.e. experimented) on the contraction of roots, you were so kind as to show much interest in, during my visit to you last year.

You can not only see the wrinkles in the bark of the roots, but very often even the oldest, central vessels of the wood are wrinkled by being contracted. The active cause of the contraction lies in the parenchymatical cells of the wood and the bark; the woody elements are only an impediment to the contraction. For this reason the roots of herbaceous plants have so much parenchym, and so often fibres and vessels.

The parenchym contracts by absorbing water. If part of a young root is put into water, it becomes shorter and thicker; you may see the cells doing the same if very thin pieces of the parenchym are put into water under the microscope. Cells and parts of tissue contract in a few minutes, the whole roots in some hours. The contraction is generally about 5%. If a root fades, it becomes flat and longer; so it is when it is killed, or when the turgor is annulled by strong solutions of salt. The contraction is caused by an increase of the turgor.

The temporary increase of the turgor must affect the growth of the cells, they must become thicker and shorter by growing.

As soon as my observations will be published, I will send them to you, but I feel that it will last long (i.e. will take some time).

With many thanks for the great marks of interest in my exper-

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iments, you so often showed me, I remain, dear Sir, with much respect

Yours faithfully
Hugo de Vries.

The reply of DARWIN again is unfortunately lost; HUGO DE VRIES returned to the subject with the following letter:

Amsterdam, Kerklaan 9
2 September 1879

My dear Sir,

I was very much pleased to see from your kind letter, that you have been coming to the same opinion on the causes of growth as I have been led to through my experiments. I always deferred answering you and thanking you for your kind words on my research, because I hoped to be able to make some more communications to you on this subject.

Since that time I experimented almost only with the tendrils of Sicyos, and I found some more arguments for the opinion, that the force of turgor is the true cause of the movements. It seemed to me of great interest to make out, whether the attractive power of the parenchyme for water is increased by the stimulus, or whether the extensibility of the elastic tissues becomes greater. The first is rather more probable, but it could not a priori be considered as sure.

To decide this question I cut off the upper side of the tendrils and brought the remaining portion in a solution of salt of 1%. Here they do not absorb water or loose it; they keep the curvations they took during their being cut. It is easy to cut them in such a way, that the epiderm, the collenchyme and the vascular bundles of the upper side are taken off, and that only the parenchyme remains, in connection with (i.e. attached to) the vascular bundles and collenchyme of the lower side. Tendrils that have operated in this way still remain sensitive, and are able to make very close curvations. To my opinion this fact proves that the force of turgor of the parenchyme is increased by the stimulus; at all events the elastic tissues of the upper side are not necessary for the movements.

[page] 196

I made another experiment to prove this. If you allow a tendril to make ½-1 curvation round a thin stick and then get it off and inject it with water under the airpump, you will see the curvations rapidly increase at the same moment. In a few minutes the tendril makes 3–5 turns, beginning in the point where it had touched the stick. I often made this experiment; it shows that the power of the parenchyme to grow by absorbing water is rapidly increased by the stimulus. Before the injection the cells could but slowly absorb water, after being injected they find it in abundance immediately around them.

With the movements of tendrils, the water absorbing power of the parenchyme is generally increased, for almost all movements are temporarily accelerated by injection with water. But in the described case the effect is always the most evident.

So it is the water absorbing power that plays the principal part in the growth and the movements, caused by stimulus. This power is due to some substance in the vacuoles of the cells; I hope to be able to recognize the nature of this substance (in) another year.

According to your wish, that I should publish in the course of the winter, I have already begun to write, and hope to finish before the end of our summer holidays.

If I were allowed to combine the results of this investigation with that of my experiments on roots, I should be led to say, that growth of cells and organs chiefly depends upon two causes: the extensibility of the cell walls and the water absorbing power of the contents of the cells. If the extensibility of the cell wall is different in various points or in various directions, the form of the cells and organs will change; so may grow the hairs, fibres, ramificated cells, cylindrical cells; the potatoes may be formed by the thin stolones. Then the force of turgor causes the rapidity of growth; it depends on the quantity of water, the light, the gravity etc. and causes the etiolement, the geotropical and heliotropical curvations, and the movements of the junctures by which many leaves and branches of inflorescenses are attached to their stems.

It seems to be quite clear, that both the force of turgor and the extensibility of the cell walls are regulated by the protoplasm. Do you think these considerations probable? Many kind thanks for your communications on the roots of Lychnis Githago. I am sorry we

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have no young specimens in our garden, so that I am not able to see the ridges. Sincerely thanking you again for your kind letter, I remain

Dear Sir,
Yours very faithfully
Hugo de Vries.

This last letter is of exceptional interest on account of the generalization which Hugo DE VRIES presents in the last part of the letter. He probably never presented these views in print.

Hugo DE VRIES published a preliminary account of this work ³³) which is dated 29 November 1879; after the presentaion of the letters of Hugo DE VRIES, it does not seem necessary to review this and some later publications on the subject.

DARWIN again replied in a letter which is lost; Hugo DE VRIES replied in the following letter:

Amsterdam, 10 Sept. 1879

My dear Sir!

I thank you very much for your kind letter, and for the seeds of Lychnis Githago, which I propose myself to sow next spring. I have finished my experiments on tendrils for this year, and hope to continue them next summer. In the course of this winter I hope to be able to send you a copy of my paper on tendrils.

In respect to the movements of Oscillaria, I am sorry to say that I have never studied them so exactly as to have an opinion on their mechanism, which seems to be very difficult to recognize.

You ask me whether I feel sure that the cell walls have not a power of contraction. I am quite sure that they often have this power, but only in cells that are extended by their turgescence. Such cells will contract by losing water. I suppose that the cells in the tentacles of Drosera are turgescent, and that those of the outside draw water from those of the inner side; thereby the first ones will extend themselves, the last ones will contract. For this is the case with tendrils. I am not able to say whether the cause of the movements of the tentacles of Drosera is the same as it is with tendrils, and my plants of Drosera are now too old to make an experiment.

[page] 198

I should be much obliged to you if you would thank your son Francis for the kind words he added to your letter.

With much respect, I remain
Dear Sir,
Yours very truly
Hugo de Vries.

After his visit to DARWIN on the fourteenth of August 1878, DE VRIES had placed Charles DARWIN on his mailing list. This list had been preserved; it shows that DARWIN received: Über die Verkuürtzung pflanzlicher Zellen durch Aufnahme von Wasser³⁴), Ueber die inneren Vorgänge bei den Wachsthumskruümmungen mehrzelliger Organe³⁵) and Ueber die Bedeutung der Pflanzensäduren für den Turgor³⁶).

On the sixth of November 1880, the results of DARWIN'S investigations in the field, in which he shared interest with DE VRIES, appeared in his book: The power of movement in plants. In addition to the reports on the work of DE VRIES in Würzburg, DARWIN cited the three short papers which were mentioned above. The subject matter of these papers had been communicated to DARWIN already in the letters for the largest part. A later, more extensive discussion ³⁷) was sent to DARWIN at a later date.

Hugo DE VRIES received a complementary copy of DARWIN'S book, which is still preserved in the library of the Hortus Botanicus in Amsterdam. It bears the dedication: From the author with very kind regards. Hugo DE VRIES acknowledged it with the following letter:

Amsterdam, 8 Dec. 1880

Dear Sir!

I am very much obliged to you for your great kindness of sending me your work on the Power of Movement in Plants, which I have read with the greatest interest. I was much pleased to learn by your experiments that circumnutation is a general phenomenon in all growing plants, and that it is the basis of most of the other movements of vegetable organs. The little oscillations you describe in these circumnutating movements seem to remind of the stossweise Aen-

[page] 199

derungen des Wachsthums of Sachs. For if it is allowed to assume that these little shocks do not occur at the same time on all sides of the growing organ, the result must be a similar movement as those, described by you. I was especially interested by your experiments on the movements and the curious sensitiveness of the roots and plumules of young seedling plants, which I hope to repeat as soon as I shall have an occasion, for I desire very much to observe myself these interesting and unexpected phenomena.

I always remember the great pleasure I had in repeating the experiments, described in your work on insectivorous plants, with all those species, which I could procure either in botanical gardens or on excursions. By so doing I not only obtained a better knowledge of the subject, but often had the opportunity of showing these phenomena to others. And now I always cultivate some Utricularia, Drosera and Pinguicula, so as to be able to show their insectivorous habits to my students every year.

Your considerations on the embryology of leaves remembered (i.e. reminded) me of the curious case, afforded by the young plants of Sium latifolium, which have their leaves divided in a much higher degree than the pinnated leaves of other plants, and so show their descent from an Umbelliferous type with highly divided leaves. So your experiments and remarks on the danger occasioned by the radiation during cold nights, suggested to me that perhaps the hairs of plants might in many cases have been acquired for the same purpose as the sleeping movements, and that this supposition would account for the circumstance that so many leaves are covered with hairs when young and loose them when growing older. In your work, you often speak of my papers on the same subject, and I am much indebted to you for your kind judgment on them, which will be a stimulus to me in endeavouring to contribute my part to the advancement of science.

With many thanks
Yours sincerely
Hugo de Vries.

The last glimpse of DARWIN'S opinion on the work of Hugo DE VRIES is found in a letter to Julius WIESNER of 4 October 1881, in which he writes ³⁸):

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I am very glad that you will again discuss the view of the turgescence of cells being the cause of the movements of parts. I adopted de Vries' views as seeming to me the most probable, but of late I have felt more doubts on this head.

It is curious that Hugo DE VRIES never commented on two books which appeared between the Climbing Plants and the Movements of Plants, which are the Cross and Self fertilization (1876) and Forms of Flowers (1877). It is almost certain that he acquired them immediately after publication, but it seems unlikely that DARWIN sent him complimentary copies; they are not among the DE VRIES books in the Hortus Botanicus in Amsterdam.

Of DARWIN'S last book however, On the Formation of Vegetable mould through the action of worms, DE VRIES received a complimentary copy, for which he thanked DARWIN in the following letter:

Amsterdam, 15 Oct. 81

Dear Sir,

I have been very much interested by your volume on the formation of vegetable mould through the action of worms, which you had the great kindness of sending me. I had often observed the frequent occurrence of these worms and their castings and was acquainted with (them) on account of Henle's paper, but had not the least suspicion of the prominent part, worms have taken in the formation of vegetable mould, as I learn from your observations.

After reading the first chapter of your book, I have been attending to the habits of worms, and had the good fortune of repeating some of your interesting observations, but it seems that time and weather are not now favourable to these experiments.

For some time I have been studying the causes of the variations of animals and plants, as described in your treatise on the variations of animals and plants under domestication, and have endeavoured to collect some more facts on this theme.

In your origin of species you have promised a volume on the variations of animals and plants in the wild state and I very much hope that some day you will have the kindness of fulfilling this

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promise and of making us acquainted with the results of your investigations on this most interesting subject. I have always been especially interested in your hypothesis of Pangenesis and have collected a series of facts in favour of it, but I am sure that your promised publication will contain much more evidence on all such points, as I would for many years be able to collect.

With many thanks for your kind
kind present
Yours sincerely
Hugo de Vries.

From the correspondence of DARWIN and DE VRIES it does not appear that DE VRIES was influenced by actual personal contact with DARWIN. Only the research on Sicyos was directly inspired by DARWIN. In spite of the large difference in age, their relationship was more a relationship of equals than a relationship of master and pupil.

Hugo DE VRIES has however always maintained that his pangenesis theory and his mutation theory were inspired by DARWIN.

The final paragraph of the last letter of DE VRIES to DARWIN inaugurates the change in the field of the research of DE VRIES which started at this time. The research on mechanical physiology was slowly abandoned and replaced by research on heredity. Starting about 1880, or perhaps even earlier, DE VRIES collected and studied all material on the subject of heredity and variation he could find, as a preparation for a research program in this field. Hence it is easy to understand that DE VRIES was interested in further books by DARWIN on the subject of variation. He probably never obtained a reply of DARWIN, who died on the nineteenth of April 1882.

It was not in the Origin of Species that DARWIN had promised a volume on the variation of animals and plants in the wild state, but in Animals and plants under domestication. DARWIN had however abandoned the idea of writing this book already in 1875, as DE VRIES might have realised if he had studied the introductions of the two existing editions of Animals and Plants under Domestication closely and compared them.

In the introduction to the first edition (1868, vol. I, p. 4), DARWIN says:

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In a second work I shall discuss the variability or organic beings in a state of nature; namely, the individual differences presented by animals and plants, and those slightly greater and generally inherited differences which are ranked by naturalists as varieties or geographical races.

In the second edition, which was published seven years later, DARWIN qualified this statement somewhat (1875, vol. I, p. 4), by adding after the word "discuss": "if time and health permit".

Some pages farther, in the introduction to the first edition DARWIN speaks even of two new books, but they are no more mentioned in the introduction to the second edition.

About the first of these, DARWIN wrote (1868, vol. I, p. 8):

In a second work, after treating of the Variation of organisms in a state of nature, of the Struggle for Existence and the principle of Natural Selection, I shall discuss the difficulties which are opposed to the theory …

and he continues this paragraph with giving a list of the difficulties which he was planning to discuss.

In the second edition, this paragraph is changed into (1875, vol. I, p. 8);

The arguments opposed to the theory of Natural Selection have been discussed in my Origin of Species as far as the size of that work permitted …

and this phrase is followed by the identical list of difficulties which had been given in the first edition.

The second additional volume was announced by DARWIN with the following words (1868, vol. I, p. 9):

In a third work I shall try the principle of Natural Selection by seeing how far it will give a fair explanation of the several classes of facts, just alluded to.

In the second edition of Animals and plants under Domestication this phrase is not found at all any more.

It is thus clear that in 1875 DARWIN had, for practical purposes,

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abandoned the idea of writing the two promised supplementary volumes, although he left the possibility of changing his mind open by retaining the first reference, only qualifying it by adding: "if time and health permit." But all this would only be clear to the reader who took the trouble of comparing both editions.

When DE VRIES quoted the Animals and plants under Domesticateion in his dissertation of 1870, he obviously used the first edition, it is unlikely that he owned the book at that time. He quotes the book repeatedly in his later works, these quotes are always to the second edition of 1875. It is quite certain that he bought this edition at the time it was published. It must have been what lingered in his mind after reading this second edition what he had in mind when he asked DARWIN whether he might still look forward to the "additional volume".

The literature study on subjects of heredity which DE VRIES made during the eighties yielded, among others, a series of 19 papers on heredity and variation in a dutch popular agricultural journal, an almost book length review of the subject. The most important result however was his book, the Intracellulare Pangenesis (Jena, 1889), perhaps the most important book DE VRIES ever wrote. In this book, DE VRIES gave a critical review of the current theories of heredity and introduced his own view on the subject, his pangenesis theory.

Hugo DE VRIES has always claimed that his pangenesis theory was based on DARWIN'S views as formulated in chapter 27 of the Animals and plants under Domestication, with the title: Provisional hypothesis of pangenesis. According to a discussion of DE VRIES in a contemporary paper ³⁹), it was possible to reduce DARWIN'S doctrine to the following two propositions ⁴⁰):

1) In each germ cell, each hereditary characteristic of the entire organism is represented by separate material particles. These particles multiply in such a way at cell division, that they are usually transferred to all succeeding cells.

2) In addition, all cells of the body produce such particles, during their growth as well as in their maturity; these so called gemmules are transported to the germ cells and are able to transfer properties to them, which they might still lack.

In his own theory, Hugo DE VRIES retained the first of these propositions. The material particles, found in the germ cells, which define

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the properties of the organism, he called pangenes, in honor of DARWIN'S hypothesis of Pangenesis.

The second proposition was however rejected by DE VRIES. Consequently, DE VRIES could not use the term gemmules as a name for his particles, this term had already a specific and different meaning.

Actually, the two propositions in which DE VRIES tried to summarize DARWIN'S doctrine, give a distorted picture thereof. DARWIN'S emphasis is on the gemmules, which are "thrown off" by all cells at all times as long as the cells are still changing. The gemmules move freely through the body of the organism in which they are formed and tend to congregate at places where a new individual is likely to develop, in buds or in the sexual cells. DARWIN does not say clearly whether the germ cells do already have a population of particles when they are formed, to which population the gemmules are added, or whether they are made up entirely from gemmules. Therefore the first proposition of DE VRIES is too positive, but by formulating DARWIN'S doctrine this way, he managed to salvage one half of DARWIN'S theory.

The pangenes of DE VRIES were material particles, which represent or define the characteristics of the organism. The significance of the hypothesis of DE VRIES became apparent after the rediscovery of MENDEL;S laws and even more so after the role of the chromosomes became known. This significance was not generally recognized shortly after the rediscovery of MENDEL'S laws, witness the fate of Hugo DE VRIES' term pangene, which is actually the gene of today. The substitution of gene for pangene was made by JOHANNSEN, because he felt that the word pangene expressed too much. In the first edition of his textbook on genetics ⁴¹), JOHANNSEN says ⁴²):

The word pangene, which was coined by DARWIN, is most generally used for Anlage. However, the word pangene is not a happy choice, since it is a combination which contains the roots Pan (all, everything) and Gen (to become). Only the meaning of the last one should be considered, only the simple idea that through "something" in the gametes a characteristic of the developing organism is or can be defined or partly defined should be expressed. No hypothesis on this "something" should here be formulated or supported. Therefore it seems the simplest thing to do to extract from DARWIN'S well known word the root gene, which alone interests us and use this

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word instead of the bad and ambiguous word Anlage. We will therefore say gene and genes for pangene and pangenes.

The german word Anlage has here only a vague meaning, something like the origin, starting point, definition of a specific character. JOHANNSEN proposed to substitute the word gene for pangene specifically to introduce this vagueness and get away from the concrete idea of material particles which DE VRIES described with his pangenes.

When at about the same time investigators started to locate the genes in the chromosomes, and it thus appeared that they were indeed material, as DE VRIES had assumed, the nomenclature gene was retained.

Thus the word gene, which is perhaps the most important concept in the science of genetics, keeps the memory of both DARWIN and DE VRIES alive.

In the cited paragraph, JOHANNSEN erroneously credited DARWIN with coining the wordt pangenes in the more specific sense. It seems that JOHANNSEN did not know the Intracellular pangenesis; it is never mentioned in his book. In the second edition (1913, p. 143) he showed however that he was better informed by changing his first sentence into ⁴³):

The word pangene, introduced by DE VRIES, following DARWIN, is most generally used for Anlage.

In his later life, Hugo DE VRIES was often asked whether he had known DARWIN personally. It seems that in his replies, he never volonteered much information. After 1900, DE VRIES was totally committed to his research in genetics and did not count his previous work in plant physiology very highly. And therefore, he did not consider his correspondence with DARWIN and his only visit, very important. And indeed, it is regrettable that the relationship had to be broken, just at the point where they were starting to discuss the topic, both were most interested in.

ACKNOWLEDGEMENTS

I express my gratitude to Professor Theo J. STOMPS, who continued the work of Hugo DE VRIES at the University of Amsterdam for his

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kind interest in my work in general and especially for copying the three letters, in which Hugo DE VRIES describes his visit to Charles DARWIN. The portrait of SACHS and his students was also supplied by Prof. STOMPS.

I am also indebted to Professor Bert James LOEWENBERG of Sarah Lawrence College, Bronxville, New York, for his kindness in sharing the results of his researches in England, even supplying photostats of the DARWIN letters. Without his assistance, it would not have been possible to present so complete a picture of DE VRIES' relations with DARWIN.

Finally, I thank Mrs. Nora BARLOW for valuable information and Mr. Robin DARWIN Esq. for permission to publish some of the letters, presented in this paper.

South Pasadena

P.W. VAN DER PAS

California

1) This paper was originally intended to be published in the Festschrift, offered to Professor Frans Verdoorn on the occasion of his 60th birthday; it was unfortunately however not ready at the time of this happy event.
2) The dissertation appeared in the dutch language only; the actual title is: De invloed der temperatuur op de levensverschijnselen der planten. 's-Gravenhage, M. Nijhoff, 1870.
3) Hugo DE VRIES, Ueber einige Ursachen der Richtung bilateral symmetrischer Pflanzentheile, Arbeiten des Botanischen Instituts zu Würzburg, I (2), 223–277, 1872.
4) Hugo DE VRIES, Ueber das Welken abgeschnittener Sprosse. Ibid., I (2), 287–301, 1873.
5) Die genauere Erkenntnis der Abhängigkeit der Verminderung der Leitfähigkeit von verschiedene Umstände, und die Entdeckung der wahren Ursache, muss späteren Untersuchungen vorbehalten bleiben.
6) Hugo (von) MOHL, Ueber den Bau und das Winden der Ranken und Schlingpflanzen, Tübingen, Heinrich Laub, 1827.
7) Charles DARWIN, On the movements and habits of climbing plants, Journal of the Linnean Society of London, IX, 1–118, 1865.
8) Hugo DE VRIES, Langenwachsthum der Oben- und Unterseite sich krümmender Ranken, Arbeiten des Botanischen Instituts zu Würzburg, I (3), 302–316, 1873.
9) Hugo DE VRIES, Zur Mechanik der Bewegung von Schlingpflanzen, Ibid., I (3), 317–342, 1873.
10) Hugo DE VRIES, Ueber die Dehnbarkeit wachsender Sprosse, Ibid., I (4), 519–545. 1874.
11) Charles DARWIN, The movements and habits of climbing plants, London, J. Murray, 1875.
12) The papers of 1873, see notes 8 and 9.
13) The original of this letter and the originals of the other seven letters of Ch. DARWIN to Hugo DE VRIES which are published in this paper, are preserved

[page] 207

in the University Library Cambridge, England. The translation of the two letters in the german language is by the author of this paper. The original text of this letter is as follows:

Würzburg, 7 Nov. 75

Hochverehrter Herr:

Empfangen Sie meinen besten Dank für die grosse Freundlichkeit, mir die zweite Auflage Ihres Werkes über Climbing Plants zu schenken. Noch mehr aber danke ich Ihnen für die freundliche Anerkennung meiner beiden Aufsätze über diesen Gegenstand, welche keinen Anspruch machen sollten auf eine so grosse Ehre, als Sie ihnen zukommen lassen.

Erst vor wenigen Tagen, als ich von einer längere Reise, nach Würzburg zurück kam, fand ich Ihr wertvolles Geschenk hier vor, und jetzt mache ich mir ein Vergnügen daraus, es genau durchzustudieren. Für die viele neue Belehrung, welche ich hierin, so wie stets in Ihren Werke finde, bringe ich Ihnen meinen aufrichtigsten Dank.

Erlauben Sie mir diese Gelegenheit zu benutzen, Ihnen speciell meine Bewunderung für Ihr letztes Werk, über Insektenessende Pflanzen auszusprechen. Ich hatte die Gelegenheit in diesem Sommer fast alle Hauptversuche mit den verschiedenen Pflanzen, theils bei Hofrat Sachs, theils selbst nach zu machen, und mich dabei nicht nur von der Richtigkeit Ihrer Beobachtungen und Folgerungen zu überzeugen, als zumal diese eingehender würdigen und bewunderen zu lernen.

Sie tragen Bedenken gegen die Auffassung dass bei den Ranken der Unterschied im Längenwachsthum der beiden Seiten die Ursache der Krümmungen sei. Ich muss gestehen, dass Sie hier einen schwachen Punkt getroffen haben. Meine Versuche beweisen direct nur, dass diese Krümmungen gewöhnlich von einer solchen Aenderung des Langenwachsthums begleitet sind, und ich gestehe dass es eine rein theoretische, für den speciellen Fall nicht bewiesene Auffassung ist, dass ich diese Wachsthumsänderung im Eingang zu meiner Arbeit als die Ursache der Krümmungen hingestellt habe. Ihre Auffassung, dass die Krümmung eine andere Ursache habe, und selbst den Unterschied in der Wachsthumsgeschwindigkeit herbeiführe, scheint mir ebenso berechtigt als die andere, und die von Ihnen angeführten Thatsachen sprechen sehr zu Ihrem Gunsten.

Ihr Bedenken trifft aber, wie ich meine, sämmtliche bis jetzt sogenannte Wachsthumskrümmungen in gleichem Maasse, und ich möchte deshalb kein Urtheil aussprechen, bis die Sache von einem allgemeineren Gesichtspunkt bearbeitet ist.

Indem ich nochmals meinen warmen Dank für Ihre freundliche Gesinnung gegen mich ausspreche, empfehle ich mich höflichst und zeichne

Mit wahrer Hochachtung
Ihr dienstf. Diener
Hugo de Vries

14) E. PRINGSHEIM, Julius Sachs, Jena, Gustav Fischer, 1932. See p. 276.
15) Vor etwa fünf Wochen schickte mir DARWIN sein neuestes Werk über die Insectivorous Plants, und zwar noch bevor es im Buchhandlung erschien. Ich fand soviel Merkwürdiges darin, dass ich sofort Drosera aus München, Dresden, Darmstadt, Holland, Jütland, Kiel usw. kommen liess und Dionea, Nephentes. Cephalotus kaufte; de Vries verschaffte mir Utricularia und Pinguicula; und so sah ich mich im Stande die wichtigsten Beobachtungen zu wiederholen und

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gestern noch schnell einen Vortrag in der Physikalischen Gesellschaft mit Demonstrationen halten zu können, der viel Bevölkerung herbeizog.
16) The original text is as follows:

Würzburg, 17 Nov. 75

Hochverehrter Herr,

Empfangen Sie meinen besten Dank für Ihren letzten Brief. Ich weiss nicht ob ich im nächsten Sommer wieder die Gelegenheit haben werde, meine Untersuchungen der Rankenpflanzen fortzusetzen… sollte dies aber der Fall sein, so werde ich es als meine erste Aufgabe betrachten, die Thatsachen, welche Sie in Ihrem Werke hervorgehoben, selbst zu beachten.

Hoffentlich werden Echinocystis lobata und Passiflora gracilis bis dahin zu beschaffen sein, doch denke ich, dass auch andere, sehr reiszbare Ranken, das nämliche zeigen werden. Wahrscheinlich werden dann wohl bei allen Krümmungen von Ranken die Beziehungen des Langenwächsthums zu der Krümmung anders aufzufassen sein, als ich es früher gethan habe.

Mich Ihnen höflichst empfehlend,

Hochachtungsvoll,
Ihr dienstfr. Diener,
Hugo de Vries.

17) Hugo DE VRIES, Untersuchungen über die mechanischen Ursachen der Zellstreckung, ausgehend von der Einwirkung von Salzlösungen auf den Turgor wachsender Pflanzenzellen, Leipzig, Wilhelm Engelmann, 1877.
18) Hugo DE VRIES, Ueber das Erfrieren der Rüben und Kartoffeln, Leopoldina, 14, 103–108, 1878.
19) Hugo DE VRIES, De ademhaling der planten, Haarlem, Tjeenk Willink, 1878.
20) The originals of these letters are lost. Contemporary copies of them are preserved among the DE VRIES papers at the Hortus Botanicus (now Hugo DE VRIES laboratory) in Amsterdam. They are written in the dutch language; the translation is by the author of this paper.
21) The original text is as follows:

Buecker's Hotel, London.
Christopher street
9 Aug. 78

Lieve grootmama

Voor eenige dagen heb ik aan Darwin geschreven om hem te vragen of en wanneer het hem schikken zou mij te ontvangen. Ik zond den brief aan zijn adres te Down, waar zijn zoon Francis mij gezegd had, dat hij zijn zou. Gisteren ontving ik antwoord van Darwin, waaruit bleek dat Fr. D. mij verkeerd had ingelicht. D. is niet tehuis, maar te Dorking waar hij nog eenige weken blijft. Het spijt mij wel, want ik had gaarne niet alleen D. gesproken, maar ook de plaats gezien waar hij zoveel voor de wetenschap gedaan heeft. Ik heb nu naar Dorking geschreven of het hem schikt mij daar te ontvangen. Van even groot belang als het bezoek aan Darwin, was voor mij met Hooker en Thiselton Dyer, de directeuren van de Botanical Gardens te Kew kennis te maken. Ik heb daarom aan den laatsten geschreven, hij is de schoonzoon van Hooker, den beroemden botanicus, dien u natuurlijk van naam kent, al was het alleen maar omdat hij de eerste geweest is, die voor eenige jaren de opmerkzaamheid van de geheele beschaafde wereld op de levenswijze der insektenetende planten vestigde; omstreeks

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een jaar vóór Darwin's boek over dit onderwerp verscheen. Thiselton Dyer antwoordde mij zeer beleefd en inviteerde mij voor heden Vrijdagavond ten eten; hij schreef mij, dat ik Hooker bij hem zou ontmoeten; dit was meer dan ik had durven hopen, ik had eenvoudig gedacht mij door hem aan Hooker voor te laten stellen. Ik ging tegen drie uur naar Kew, maakte een bezoek bij den Heer Dyer, om hem te zeggen dat ik aan zijne invitatie gaarne gevolg zou geven; hij zeide dat het hem speet mij nu niet door den tuin te kunnen begeleiden, doch gaf mij een wegwijzer door de kassen, waarmede ik toen van 3–7 uur de kassen en den tuin bezichtigd heb. Tegen 7 uur ging ik naar zijn huis, een zeer lieve villa dicht bij het station. Ik ontmoette daar, behalve zijn vrouw en Sir Joseph Dalton Hooker nog een tante van Dyer die er logeerde en een Mr. Armstrong uit Londen. Ik genoot de eer Mevrouw naar de eetzaal te leiden en tusschen de beide dames te zitten. Een uitvoerig verslag van dit diner zal ik u mondeling wel eens doen. Natuurlijk werd ik weer voor een zoon van Prof, de Vriese gehouden. Hooker vertelde mij veel van zijne bezoeken in Holland, voor lange lange jaren, en van de botanici die toen leefden. Verder van insektenetende planten en allerlei andere onderwerpen. Na den eten dronken wij koffie en thee en om 10 uur vertrok ik per North London railway herwaarts. Het trof heel gelukkig dat ik juist heden gegaan ben, want morgen gaat Hooker op reis, ik vind het zeer vriendelijk van Mr. Dyer dat hij mij vandaag gevraagd heeft, ofschoon hij eigenlijk geen tijd had zich heden met mij op te houden. Hij heeft mij voorgesteld morgen weer te Kew te komen en het nieuwe gebouw voor het herbarium te bezichtigen, alsook zijn laboratorium, waar ik ook met Burdon Sanderson, eveneens een bekend plantenpsycholoog zal kennis maken. Ik ga er dus morgen weer heen. 'T is reeds laat, bijna 12 uur en ik zal dus hier maar eindigen en u dezen morgen maar toezenden.

Hart. gr. uw kl.
wg. Hugo de Vries.

22) Nora BARLOW, The autobiography of Charles Darwin, London, Collins, c. 1958. See p. 110.
23) William CARR, Sir Thomas H. Farrer, Dict. Nat. Biog., 22, 626–627, 1909.
24) ANON., Sir John Scott Burdon Sandersen, Enc. Britt., XI ed., 4, 811, 1910.
25) The original text is as follows:

London, 14 Aug. 1878

Heden ben ik dan bij Darwin geweest. Ik ben blij dat het gebeurd is, en ik moet zeggen, Darwin was zoo bijzonder hartelijk en vriendelijk, dat ik het toch wel aardig vind, dat ik bij hem geweest ben. Ik ben er van 2–4 uur geweest; om 3 uur moest hij een half uur rust nemen, om zich niet te veel te vermoeien, en toen ging ik met een neef van hem, Mr. Farrer, zoolang door hun buiten wandelen. Eerst kwam ik in de familie, Mevr. Darwin, dochter en eenige dames Farrer; daarna met Darwin alleen op zijne kamer en toen wat met hem gewandeld tot 3 uur; omstreeks ¼ voor 4 koffie gedronken met D. en Mevr. Farrer waarna ik door hun rijtuig naar den trein gebracht werd. Het gesprek vlotte nogal; zij spraken allen langzaam en duidelijk en lieten mij ook den tijd om rustig uit te spreken, zoodat ik mijzelf in 't Engelsch spreken erg meeviel, en dat, hoe gebrekkig het ook was, de dames meenden vrijheid te hebben mij een compliment te maken, alsof ik goed Engelsch sprak, doch ten onrechte. In den tuin waren kassen met perziken en druiven. D. vertelde mij een lang verhaal van perziken en bood er mij meteen een aan om te eten, overheerlijk! In onze wetenschappelijke gesprekken heerschte dikwijls diezelfde lachmanie, die je bij Sachs

JANUS, LVII 14

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gezien hebt; Sachs lacht altijd door, Darwin niet zooveel maar even opgewekt. Hij interesseerde zich zeer voor wat ik in den laatsten tijd gedaan had; het trof, dat het juist onderwerpen waren, die hij ook had willen bestudeeren, maar hij was er niet toe gekomen. Hij vertelde allerlei van zijn eigen onderzoekingen, bijv. toen hij pas al die eigenaardige inrichtingen in de bloemen der Orchideeën voor de bestuiving door insekten ontdekt had, lang voordat hij hierover iets publiceerde. Hij vertelde alles aan Huxley die antwoordde: "And, my good fellow, would you have me to believe this all?" Dit is lang geleden, thans weet iedereen dat het waar is. Hij vertelde mij, dat hij moeilijk talen kon leeren en dat zijne vrouw en zoons hem hielpen in het lezen van Duitsche boeken, maar hoe of het kwam, dat ze Pfeffer's boeken nooit konden ontcijferen; of hij zulk moeilijk Duitsch schreef? Mijn opinie was, dat ik hem meest ook niet begreep en dat het niet aan het Duitsch lag, maar aan de onhelderheid van stijl, wat Darwin grootelijks verheugde. Of ik wel eens wat aan bestuiving van bloemen door insekten gedaan had? Wat de pruiken aan den pruikenboom toch wel voor doel hebben? dat er zoo weinig plantenphysiologen in Engeland zijn; dat 't jammer is dat wij niet allen Engelsch schrijven. Hij is erg ingenomen met Sachs, over wien wij veel gepraat hebben. Hij legt een voetkussen op een stoel, eer hij er op gaat zitten, omdat hij hoofdpijn krijgt als hij laag zit — de arme man!

Mr. Farrer vertelde mij, dat hij vandaag bijzonder wel en opgewekt was, en dat ik het best trof. Mevr. D. paste altijd erg op hem en wil nooit toestaan, dat hij zich te zeer vermoeit, dan stuurt zij hem eenvoudig naar bed. Hooker had eens tegen D. gezegd, dat hij zonder zulk eene vrouw nooit dat voor de wetenschap had kunnen doen, wat hij gedaan heeft. Darwin was erg vriendelijk, hij zeide, dat hij mij wel zou geschreven hebben om naar Dorking te komen, maar dat hij vreesde dat ik dit er niet voor over zou hebben, om hem te bezoeken! Al te beleefd, vindt ge niet? Vrijdag vertrekt Darwin naar eene veel verder gelegen plaats; het trof dus nogal goed, dat ik hem juist dezer dagen spreken kon. Hij heeft erg diepliggende oogen en daarenboven nog sterk vooruitspringende wenkbrauwen, veel meer dan ik naar zijn portret gedacht had. Hij is lang en erg mager, magere handen; hij loopt langzaam en niet zonder stok, en moet van tijd tot tijd stilstaan, is erg bang voor kou en moet in één woord vreeselijk op zijn gezondheid passen. Hij praat vreeselijk levendig en opgewekt en hartelijk, maar niet te ras en heel duidelijk.

Het is merkwaardig, zoo gauw als je met zoo iemand op je gemak voelt, als hij vriendelijk en hartelijk is. Hoe geheel anders met Hooker en Dyer, die waren koud en lieten mij onverschillig, maar met Darwin heb ik werkelijk genoten en gevoel mij weer veel opgewekter dan de laatste dagen. Het is zoo'n genot te zien, dat zoo iemand werkelijk belang in je stelt en dat, wat je zoo gevonden hebt hem werkelijk iets schelen kan.

26) Asa GRAY, Field, Forest and Garden botany, a simple introduction to the common plants, east of the Mississippi, New York and Chicago, Ivison, Blake-man and Taylor, c. 1868. See p. 84.
27) The original text is as folows:

Bueckers Hotel
Christophor street
London 14 Aug 1878

Lieve grootmama

Heden is het eigenlijke doel van de reis bereikt, en ik haast mij u te schrijven. Ik heb Darwin gesproken. Ik heb hem bezocht en ben zoo vriendelijk, ja

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zoo hartelijk door hem ontvangen, als ik mij in de verte niet had durven voorstellen. Doch laat ik mijn verhaal met het begin beginnen. Hij had mij geschreven met welke treinen ik kon komen en weggaan, en de keus van den dag aan mij overgelaten. Ik had geantwoord dat ik heden koos. Om 11 uur vertrok ik met den trein en kwam te 1 te Gomshall aan; hier dejeuneerde ik eerst even in 't dorp, huurde toen een rijtijuigje, dat mij naar Abinger-Hall, het buiten van den heer Farrer bracht, waar Darwin logeerde. De hr. Farrer schijnt een zwager van D. te zijn, ten minste zijn zoon zei „uncle Charles‟ tegen Darwin. Ik gaf mijn kaartje voor D., werd binnen gelaten en zeer vriendelijk ontvangen. Hij stelde mij voor, eerst met de familie kennis te maken, om daarna wat alleen met hem op zijn studeerkamer te praten. Ik werd voorgesteld aan Mevr. D. en haar dochter, verder aan Mevr. Farrer, zoon en dochter (de Hr. Farrer was "on business" in London). Wij praatten een poosje over allerlei, het buiten (dat zeer groot en prachtig is), de omstreken (ook zeer mooi), politiek, mijn reis, enz. Daarna nam D. mij met zich naar zijne kamer en toen liep het gesprek over wetenschappelijke dingen. Eerst over ranken, in aansluiting aan onze vroegere correspondentie; hij vertelde mij eenige argumenten voor zijn toen geuite meening, en vroeg of ik er nog verder aan gewerkt had. Ik had een methode uitgewerkt, om zijn vraag te kunnen beantwoorden, maar had de plant die hij mij aanbevolen had, niet kunnen krijgen, noch te Würzburg, noch te Amsterdam; ook de Hortulanus kon haar nergens krijgen. Darwin had zijn zaden van Prof. Asa Gray in Amerika gekregen en beloofde mij bij dezen geleerde om nieuwe zaden voor mij te zullen vragen.

Verder vroeg hij wat ik in den laatsten tijd gewerkt had; nu trof het dat ik juist bezig ben aan een onderwerp, dat hem zeer moest interesseeren, het kruipen van planten in den grond om zich tegen de vorst in den winter te beschermen. Dit gebeurt doordat de wortels zich verkorten, ik had dit gezien en gemeten. Het gebeurt bij rijk bebladerde eenjarige planten. D. had ergens eene opmerking gelezen dat iets dergelijks bij kiemplanten voorkomt; hij had de soorten in quaestie gezaaid om het zelf waar te nemen in dit voorjaar, maar het zaad was niet opgekomen. Hij was nu zeer verheugd van mij te hooren, dat ook grootere planten het doen, en dat dit vrij algemeen voorkomt, en een bepaald doel in het leven der plant (bescherming in den winter) schijnt te hebben. Wij praatten hier lang over. Verder allerlei over Sachs, Pfeffer, over allerlei literatuur, over het verblijf van zijn zoon Francis te Würzburg enz. te veel om alles op te schrijven. Toen stelde hij mij voor wat in den tuin te gaan wandelen, wees mij allerlei planten, waarvan wat te vertellen viel, en vertelde mij veel interessants. Toen was hij vermoeid en moest rust gaan nemen; hij mag niet lang achter elkander praten, en vooral niet zoo druk als vandaag. Hij stelde mij echter voor te blijven, en na een half uurtje met hem en familie Farrer koffie te gebruiken; tot zoolang zou de zoon Farrer met mij op het buiten rondwandelen en mij allerlei laten zien. Dit geschiedde. Aan de koffie was D. blijkbaar nog vermoeid, maar toch zeer opgewekt; hij liet zich allerlei van Holland vertellen, en vertelde mij van zijne reizen om de wereld. Toen het tijd werd om te vertrekken nam ik afscheid, en werd door het rijtuig van den heer Farrer naar het station gebracht. Ik wist allang dat D. zeer vriendelijk en goedhartig is, maar zoo vriendelijk als hij vandaag voor mij was, had ik hem toch niet gedacht. Hij was, vertelde mij Mr. Farrer, vandaag bizonder wel, maar men kon het hem toch aanzien dat hij zwak en ziekelijk was. Hij is een lang man, en zijn portret gelijkt zeer goed (naar het portret had ik gedacht

[page] 212

dat hij klein was). Later zal ik u alles wel eens uitvoerig vertellen, het is niet mogelijk alles op te schrijven.

Uw liefh. H. d. V.

PS. Maand. was Prof. Suringar hier, ik heb met hem te Kew Burdon Sanderson bezocht, die ons zeer beleefd zijn laboratorium en zijn toestellen liet zien en ons ook hun werking liet waarnemen.

28) The original of this letter is preserved among the GRAY papers in the Arnold Arboretum of Harvard University, Cambridge, Mass. A summary of this letter has been published in: B. J. LOEWENBERG, Calendar of the letters of Charles Robert Darwin to Asa Gray. Boston, Historical Records Survey, 1939.
29) The original of this letter is preserved among the DE VRIES papers in the Hortus Botanicus of Amsterdam.
30) W. VAN ITALIE-VAN EMBDEN, Sprekende portretten, Rotterdam, Nijgh en van Ditmar, 1928. See p. 69–80.
The original text is:
„Tot de erfelijkheidsleer bracht me… mijn liefde voor Darwin.‟ „Hebt u hem gekend?‟ Een glans lichtte door de oogen: „Ik ben bij hem geweest. Hij was oud; ik piepjong. Hij lag op een ruststoel; ik mocht naast hem zitten. Darwin was toen de eenige die schiep op dit gebied. Nù is het zóó uitgebreid.‟
31) Hugo DE VRIES, Over de contractie van wortels, Verslagen en mededeelingen der Kon. Ac. van Wetenschappen te Amsterdam, (2) 15, 12–16, 1880.
32) Hugo DE VRIES, Ueber die Kontraktion der Wurzeln, Landwirtschaftliche Jahrbücher, 9, 37–80, 1880.
33) Hugo DE VRIES, Over de oorzaken van krommingen bij den groei van plantendeelen, Proces-verbaal van de gewone vergadering der Afd. Natuurkunde; 1879/80, n° 5, pp. 5–7.
34) Hugo DE VRIES, Ueber Verkurtzung pflanzlicher Zellen durch Aufnahme von Wasser, Botanische Zeitung, 37, 649–654, 1879.
35) Hugo DE VRIES, Ueber die inneren Vorgänge bei den Wachsthum mehrzelliger Organe. Botanische Zeitung, 37, 830–838, 1879.
36) Hugo DE VRIES, Ueber die Bedeutung der Pflanzensäuren für den Turgor der Zellen. Botanische Zeitung, 37, 847–853, 1879.
37) Hugo DE VRIES, Over de beweging der ranken van Sicyos, Verslagen en mededeelingen der Kon. Ac. van Wetenschappen te Amsterdam, 15, 51–174, 1880.
38) F. DARWIN, More letters of Charles Darwin, London, John Murray, 1903. See. vol. II, pp. 432–433.
39) Hugo DE VRIES, Darwin's denkbeelden over de stoffelijke oorzaken der erfelijkheid, Album der natuur, 38, 73–91, 1889.
40) The original text is:
1. In iedere kiemcel is iedere erfelijke eigenschap van het geheele organisme door afzonderlijke stoffelijke deeltjes vertegenwoordigd. Deze vermenigvuldi-gen zich zÓó, dat zij bij de celdeelingen in den regel op alle volgende cellen overgaan.
2. Bovendien zonderen al de cellen van het lichaam, zoowel tijdens haren groei, als in volwassen toestand, zulke deeltjes af; deze zoogenoemde kiempjes worden naar de kiemcellen vervoerd en kunnen aan deze de eigenschappen mededeelen, die haar soms mochten ontbreken.
41) W. JOHANNSEN, Elemente der exakten Erblichkeitslehre, Jena, Gustav Fischer, 1909. See p. 124.
42) The original text is:

[page] 213

Das VON DARWIN eingeführte Wort „pangene‟ wird wohl am häufigsten statt „Anlagen‟ benutzt. Jedoch ist das Wort „Pangen‟ nicht glücklich gewählt, indem es eine Doppelbildung ist, die Stämme Pan (all, jeder) und Gen (werden) enthaltend. Nur der Sinn dieses letzteren kommt hier in Betracht; bloss die einfache Vorstellung soll Ausdruck finden, dass durch „etwas‟ in den Gameten eine Eigenschaft des sich entwickelenden Organismus bedingt oder mitbestimmt wird oder werden kann. Keine Hypothese über das Wesen dieses „etwas‟ sollte dabei aufgestellt oder gestützt werden. Darum scheint es am einfachsten, aus DARWIN'S bekannten Wort die uns allein interessierende letzte Silbe „Gen‟ isoliert zu verwerten, um damit das schlechte, mehrdeutige Wort „Anlage‟ zu ersetzen. Wir werden somit für „das Pangen‟ und „die pangene‟ einfach „das Gen‟ und „die Gene‟ sagen.
43) The original text is:
Das von DE VRIES im Anschluss an DARWIN eingeführte Wort „pangene‟ ist wohl am häufigsten statt „Anlagen"‟ benutzt worden.

P. W. VAN DER PAS

1324 Gates Place,
South Pasadena
California 91030