Origin of Species Variorum

Comparison with 1872

several members of the same class, especially if in members having very different habits of life, we may generally attribute its presence to inheritance from a common ancestor; and its absence in some of the members to .. loss through disuse or natural selection. So that, if the electric organs had been inherited from some one ancient progenitor, .. we might have expected that all electric fishes would have been specially related to each other; but this is far from the case. Nor does geology at all lead to the belief that .. most fishes formerly possessed electric organs, which .. their modified descendants have now lost. But when we look closer to the subject, we find in the several fishes provided with electric organs that these are situated in different parts of the body,—that they differ in construction, as in the arrangement of the plates, and, according to Pacini, in the process or means by which the electricity is excited,—and lastly, in the requisite nervous power (and this is perhaps the most important of all the differences) being supplied through different nerves from widely different sources. Hence in the several remotely allied fishes furnished with electric organs, these cannot be considered as homologous, but only as analogous in function. Consequently there is no reason to suppose that they have been inherited from a common progenitor; for had this been the case they would have closely resembled each other in all respects. Thus the greater difficulty disappears, leaving only the lesser yet still great difficulty; namely, by what graduated steps these organs have arisen and been developed in each separate fish. ↑ The luminous organs which occur only in a few insects, belonging to widely different families and orders, and which are situated in different parts of the body, offer a difficulty almost exactly parallel with that of the electric organs. Other similar cases could be given; for instance in plants, the very curious contrivance of a mass of pollen-grains, borne on a foot-stalk with an adhesive gland, is apparently the same in Orchis and Asclepias,— genera almost as remote as is possible amongst flowering plants; but here again the parts are not homologous. In all cases of beings, far removed from each other in the scale of organisation, which are furnished with similar and peculiar organs, it will be found that although the general appearance and function of the organs may be the same, yet fundamental differences between them can always be detected. For instance, the eyes of cephalopods or cuttle-fish and of vertebrate animals appear wonderfully alike; and in such widely sundered groups no part of this resemblance can be due to inheritance from a common progenitor. Mr. Mivart has advanced this case as one of special difficulty, but I am unable to see the force of his argument. An organ for vision must be formed of transparent tissue, and must include some sort of lens for throwing an image at the back of a darkened chamber. Beyond this superficial resemblance, there is hardly any real similarity between the eyes of cuttle-fish and vertebrates, as may be seen by consulting Hensen's admirable memoir on these organs in the Cephalopoda. It is impossible for me here to enter on details, but I may specify a few of the points of difference. The crystalline lens in the higher cuttle-fish consists of two parts, placed one behind the other like two lenses, both having a very different structure and disposition to what occurs in the vertebrata. The retina is wholly different, with an actual inversion of the elemental parts, and with a large nervous ganglion included within the membranes of the eye. The relations of the muscles are as different as it is possible to conceive, and so in other points. Hence it is not a little difficult to decide how far even the same terms ought to be employed in describing the eyes of the Cephalopoda and Vertebrata. It is, of course, open to any one to deny that the eye in either case could have been developed through the natural selection of successive, slight variations; but if this be admitted in the one case, it is clearly possible in the other; and fundamental differences of structure in the visual organs of two groups might have been anticipated, in accordance with this view of their manner of formation. I am inclined to believe that in nearly the same way as two men have sometimes independently hit on the very same invention, so natural selection, working for the good of each being and taking advantage of analogous variations, has sometimes modified in very nearly the same manner two parts in two organic beings, which owe but little of their structure in common to inheritance from the same ancestor.

Fritz Müller, in a remarkable work recently published, has discussed a case nearly parallel with that of electric fishes, luminous insects, &c.; he undertook the laborious examination of this case in order to test the views advanced by me in this volume. Several families of crustaceans include a few members which are fitted to live out of the water and possess an air-breathing apparatus. In two of these families, which were more especially examined by Müller, and which are nearly related to each other, the species agree most closely in all important characters: namely in the structure of their sense-organs, in their heart and system of circulation, in the position of every tuft of hair with which their stomachs, equally complicated in both cases, are lined, and lastly in the water-breathing branchiæ, even to the microscopical hooks by which they are cleansed. Hence it might have been expected from mere analogy that the equally important air-breathing apparatus would have been the same in the few species in both families which are thus furnished; and this might have been the more confidently expected by those who believe in the creation of each separate species; for why should this one apparatus, given for the same special purpose to a few species which are so closely similar or rather identical in all other important points, have been made to differ?

Fritz Müller then argued to himself that this close similarity in so many points of structure must, in accordance with the views advanced by me, be accounted for by inheritance from a common progenitor. But as the vast majority of the species in the above two families, as well as the main body of crustaceans of all orders, are aquatic in their habits, it is improbable in the highest degree, that their common progenitor should have been adapted for breathing air. Müller was thus led carefully to examine and describe the apparatus in the few air-breathing species; and in each he found it to differ in several important points, as in the position of the orifices, in the manner in which they are opened and closed, and in some accessory details. Now such differences are intelligible, and might even have been anticipated, on the supposition that species belonging to distinct families had slowly become adapted to live more and more out of water, and to breathe the air. For these species, from belonging to distinct families, would differ to a certain extent, and in accordance with the principle that the nature of each variation depends on two factors, viz. the nature of the organism and that of the conditions, the variability of these crustaceans assuredly would not have been exactly the same. Consequently natural selection would have had different materials or variations to work on, in order to arrive at the same functional result; and the structures thus acquired would almost necessarily have differed. ↑ On the hypothesis of separate acts of creation the whole case must remain unintelligible, and we can only say, so it is. This line of argument seems to have had great weight in leading this distinguished naturalist fully to accept the views maintained by me in this volume.

Another distinguished zoologist, the late Professor Claparède, has argued in the same manner, and has arrived at the same result. He shows that there are parasitic mites (Acaridæ), belonging to distinct sub-families and families, which are furnished with hair-claspers. These organs must have been independently developed, as they could not have been inherited from a common progenitor; and in the several groups they are formed by the modification of the fore-legs,— of the hind-legs,— of the maxillæ or lips,— and of appendages on the under side of the hind part of the body.

In the several cases just discussed, we have seen that in beings more or less remotely allied, the same end is gained and the same function performed by organs in appearance, though not in truth, closely similar. But the common rule throughout nature is that the same end is gained, even sometimes in the case of beings closely related to each other, by the most diversified means. How differently constructed is the feathered wing of a bird and the membrane-covered wing of a bat with all its fingers developed; and still more so the four wings of a butterfly, the two wings of a fly, and the two of a bettle with their elytra. Bivalve shells have only to open and shut, but on what a number of patterns is the hinge constructed, from the long row of neatly interlocking teeth in a Nucula to the simple ligament of a Mussel. Seeds are disseminated by their minuteness or by their capsule being converted into a light ballon-like envelope; or by being embedded in pulp or flesh, formed of the most diverse parts, and rendered nutritious as well as conspicuously coloured, so as to attract and be devoured by birds; or by having hooks and grapnels of many kinds and serrated awns, so as to adhere to the fur of quadrupeds; or by being furnished with wings and plumes, as diversified in shape as elegant in structure, so as to be wafted by every breeze. I will give one other instance; for the subject is worthy of reflection by those who are not able to credit that organic beings have been formed in many ways for the sake of mere variety, like toys in a shop. Some authors maintain that organic beings have been formed in many ways for the sake of mere variety, almost like toys in a shop, but such a view of nature is incredible. With plants having separated sexes, and with those in which, though hermaphrodites, the pollen does not spontaneously fall on the stigma, some aid is necessary for their fertilisation. With several kinds this is effected by the light and incoherent pollen-grains being blown by the wind through mere chance on to the stigma; and this is the simplest plan which can well be conceived. An almost equally simple, though very different, plan occurs in many cases, in which a symmetrical flower secretes a few drops of nectar, and is consequently visited by insects; and these carry the pollen from the anthers to the stigma.

From this simple stage we may pass through an in-exhaustible number of contrivances, all for the same purpose and effected in essentially the same manner, but entailing changes in every part of the flower; with the nectar stored in variously shaped receptacles, with the stamens and pistils modified in many ways, sometimes forming trap-like contrivances, and sometimes capable of neatly adapted movements through irritability or elasticity. From such structures we may advance till we come to such an acme of perfect adaptation, as has lately been described by Dr. Crüger in the case of Coryanthes. This orchid has its labellum or lower lip hollowed out into a great bucket, into which drops of almost pure water, not nectar, continually fall from two secreting horns which stand above it; and when the bucket is half full, the water overflows by a spout on one side. The basal part of the labellum curves over the bucket, and is itself hollowed out into a sort of chamber with two lateral entrances, within which and outside there are some curious fleshy ridges.

The most ingenious man, if he had not witnessed what takes place, could never have imagined what purpose all these parts served. But Dr. Crüger saw crowds of large humble-bees visiting the gigantic flowers of this orchid in the early morning, and they came, not to suck nectar, but to gnaw off the ridges above the bucket; in doing this they frequently pushed each other into the bucket, and thus their wings were wetted, so that they could not fly out, but had to crawl out through the passage formed by the spout or overflow. Dr. Crüger has seen a "continual procession" of bees thus crawling out of their involuntary bath. The passage is narrow, and is roofed over by the column, so that a bee, in forcing its way out, first rubs its back against the viscid stigma and then against the viscid glands of the pollen-masses. The pollen-masses are thus glued to the back of the bee which first happens to crawl through the passage of a lately expanded flower, and are thus carried away. Dr. Crüger sent me a flower in spirits of wine, with a bee which he had killed before it had quite crawled out of the passage with a pollen-mass fastened to its back. When the bee, thus provided, flies to another flower, or to the same flower a second time, and is pushed by its comrades into the bucket and then crawls out by the passage, the pollen-mass necessarily comes first into contact with the viscid stigma, and adheres to it, and the flower is fertilised. Now at last we see the full use of the water-secreting horns, of the bucket with its spout, and of the shape of every part of the flower! The construction of the flower of another closely allied orchid, namely Catasetum, is widely different, though serving the same end; and is equally curious. Bees visit this flower, as in the case of the Coryanthes, in order to gnaw the labellum; in doing this they inevitably touch a long, tapering, sensitive projection, or, as I have called it, antenna. The antenna being touched causes a certain membrane to rupture through its own irritability, and this sets free a spring by which the pollen-mass is shot forth, like an arrow, in the right direction, and adheres by its viscid extremity to the back of the bee. The pollen-mass is thus carried to another flower, where it is brought into contact with the stigma, which is viscid enough to break certain elastic threads, and to retain the pollen-mass, which then performs its office of fertilisation.

How, it may be asked, in the foregoing and in innumerable other and similar cases, can we understand the cause of such a wide scale of complexity and of such multifarious means for gaining the same end, both in the case of forms widely remote from each other in affinity, and with forms so closely allied as are the two orchids last described? The answer no doubt is, as already remarked, that when two forms vary, which already differ from each other even in a slight degree, the variability will not be of the same exact nature, and consequently the results obtained through natural selection for the same general purpose will not be the same. We must also bear in mind that every well-developed organism has already passed through a long course of modification; and that each modified structure tends to be inherited, so that it will not readily be lost, but may be modified again and again. Hence the structure of each part of each species, for whatever purpose used, will be the sum of the many inherited changes, through which that species has passed during its successive adaptations to changed habits and conditions of life.

Finally then, although in many cases it is most difficult even to conjecture by what transitions an organ .. have arrived at its present state; yet, considering how small the proportion of living and known forms is to the extinct and unknown,

several members of the same class, especially if in members having very different habits of life, we may attribute its presence to inheritance from a common ancestor; and its absence in some of the members to its loss through disuse or natural selection. But if the electric organs had been inherited from one ancient progenitor thus provided, we might have expected that all electric fishes would have been specially related to each other. Nor does geology at all lead to the belief that formerly most fishes had electric organs, which most of their modified descendants have lost. The presence of luminous organs in a few insects, belonging to different families and orders, offers a parallel case of difficulty. ↑ The presence of luminous organs in a few insects, belonging to different families and orders, offers a parallel case of difficulty. ↑ In all these cases of two very distinct species furnished with apparently the same anomalous organ, it should be observed that, although the general appearance and function of the organ may be the same, yet some fundamental difference can generally be detected. I am inclined to believe that in nearly the same way as two men have sometimes independently hit on the very same invention, so natural selection, working for the good of each being and taking advantage of analogous variations, has sometimes modified in very nearly the same manner two parts in two organic beings, which beings owe but little of their structure in common to inheritance from the same ancestor.

Although in many cases it is most difficult to conjecture by what transitions .. organs could have arrived at their present state; yet, considering that the proportion of living and known forms to the extinct and unknown