Origin of Species Variorum

Comparison with 1866

identically the same, yet some fundamental difference between them can always, or almost always, 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 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, on the belief that species belonging to distinct families, already differing in some characters, and which whenever they varied would probably have varied in different manners, have been slowly adapted through natural selection to live more and more out of water and to breathe the air, it is quite intelligible, and might even have been con- fidently expected, that the structural contrivances thus acquired would in each case have materially differed, although serving for the same purpose. 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. ↑

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. ↑ 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? It was shown, when we discussed the air-breathing apparatus of certain crustaceans, that the process of adaptation for any purpose may start from two or more forms already differing from each other to a considerable degree, and that in almost all cases the nature of the variability, on which natural selection has to work, will be different; consequently, the final structure gained through natural selection, though serving for the same purpose, will be different. 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 to conjecture by what transitions .. organs .. have arrived at their present state; yet, considering that the proportion of living and known forms ... is very small compared with the extinct and unknown forms, I have been astonished how rarely an organ can be named, towards which no transitional grade is known to lead. It certainly is true, that new organs very rarely or never suddenly appear in any class, as if created for some special purpose; as indeed is shown by that old, but somewhat exaggerated, canon in natural history of "Natura non facit saltum." We meet with this admission in the writings of almost every experienced naturalist; .. as Milne Edwards has well expressed it, Nature is prodigal in variety, but niggard in innovation. Why, on the theory of Creation, should there be so much variety and so little novelty? Why should all the parts and organs of many independent beings, each supposed to have been separately created for its proper place in nature, be so commonly linked together by graduated steps? Why should not Nature .. take a sudden leap from structure to structure? On the theory of natural selection, we can clearly understand why she should not; for natural selection .. acts only by taking advantage of slight successive variations; she can never take a sudden leap, but must advance by short and sure though slow steps.

Organs of little apparent Importance, as affected by Natural Selection.

As natural selection acts by life and death,—by .. the preservation of individuals with any favourable variation, and by the destruction of those with any unfavourable deviation of structure,—I have sometimes felt much difficulty in understanding the origin of simple parts, of which the importance does not seem sufficient to cause the preservation of successively varying individuals.

the same manner two parts in two organic beings, which 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 an organ could have arrived at its present state; yet, considering that the proportion of living and known forms to the extinct and unknown is very small, I have been astonished how rarely an organ can be named, towards which no transitional grade is known to lead. The truth of this remark is indeed shown by that old canon in natural history of "Natura non facit saltum." We meet with this admission in the writings of almost every experienced naturalist; or, as Milne Edwards has well expressed it, nature is prodigal in variety, but niggard in innovation. Why, on the theory of Creation, should this be so? Why should all the parts and organs of many independent beings, each supposed to have been separately created for its proper place in nature, be so invariably linked together by graduated steps? Why should not Nature have taken a leap from structure to structure? On the theory of natural selection, we can clearly understand why she should not; for natural selection can act only by taking advantage of slight successive variations; she can never take a leap, but must advance by the shortest and slowest steps.

Organs of little apparent importance .—

As natural selection acts by life and death,— by the preservation of individuals with any favourable variation, and by the destruction of those with any unfavourable deviation of structure,— I have sometimes felt much difficulty in understanding the origin of simple parts, of which the importance does not seem sufficient to cause the preservation of successively varying individuals.