But let us come to the examples. It would be interest- ing first to institute here a general comparison between plants and animals. One cannot fail to be struck with the parallel progress which has been accomplished, on both sides, in the direction of sexuality. Not only is fecunda- tion itself the same in higher plants and in animals, since it consists, in both, in the union of two nuclei that differ in their properties and structure before their union and immediately after become equivalent to each other; but the preparation of sexual elements goes on in both under like conditions : it consists essentially in the reduction of the number of chromosomes and the rejection of a certain quantity of chromatic substance.' Yet vegetables and animals have evolved on independent lines, favored by unlike circumstances, opposed by unlike obstacles. Here are two great scries which have gone on diverging. On either line, thousands and thousands of causes have com- bined to determine the morphological and functional evolution. Yet these infinitely complicated causes have been consummated, in each series, in the same effect. And this effect could hardly be called a phenomenon of "adapt- I ation": where is the adaptation, where is the pressure ' of external circumstances? There is no striking utility
P. Gufrin. Let Connaissancea aetveUes «ur U
jihmiTogames, Paris, 1904, pp. 144-148. Cf.
2ad edition, 1903, pp. 140 fT.
jiamdation ehtt Um Dolage. L'HirtdiU,
60 CREATIVE EVOLUTION iohap."
in sexual generation; it haa been interpreted in the most diverse ways; and some very acute enquirers evan regard the sexuality of tlie plant, at least, as a luxury which nature might have dispensed with.' But we do not wish to dwell on facts so disputed. The ambiguity of the teim " adapta- tion," and the necessity of transcending both the point of view of mechanical causality and that of anthropomor- phic finality, will staad out more clearly with simpler examples. At all times the doctrine of finality has laid much stress on the marvellous structure of the sense- organs, in order to liken the work of nature to that of an intelligent workman. Now, since these organs are found, in a rudimentary state, in the lower animals, and since nature offers us many intermediaries between the pig- ment-spot of the simplest organisms and the infinitely complex eye of the vertebrates, it may just as well be alleged that the result has been brought about by natural selection perfecting the organ automatically. In short, if there is a case in which it seems justifiable to invoke adaptation, it is this particular one. For there may be di8cus.sion about the function and meaning of such a thing as sexual generation, in so far as it is related to the con- ditions in which it occurs; but the relation of the eye to light is obvious, and when we call this relation an adapta- tion, we must know what we mean. If, then, we can show, in this privileged case, the insufficiency of the principles invoked on both sides, our demonstration will at once have reached a high degree of generality.
Let us consider the example on which the advocates
of finality have always insisted: the structure of such
an organ as the human eye. They have had no diffi-
' Hcbius, fleifrcwje zur Lehrt von der Fortpfianrung der GeuUch**, Jena, t8&7, pp. 203-20Giii piirticuUr. Cf. HBjtog,"Sur les phinomenea de leproductioD" (Annie biologique, 1895, pp. 707-709).
The Choice of an Example
culty in showing that in this extremely complicated ap- paratus all the elements are marvelously co-ordinated. In order that vision shall operate, says the author of a well- known book on Final Causes, "the sclerotic membrane must become transparent in one point of its surface, so 3 to enable luminous rays to pierce it . . . ; the cornea must correspond exactly with the opening of the socket behind this transparent opening there must be I refracting media . . .; there must be a retina' at the extremity of the dark chamber . . .; perpendicular to the retina there must be an innumerable quantity of trans- parent cones permitting only the light directed in the hne of their axes to reach the nervous membrane,"' etc. etc. In reply, the advocate of final causes has been invited to assume the evolutionist hypothesis. E\'erytliing is mar- velous, indeed, if one consider an eye like ours, in which thousands of elements are coordinated in a single function. But take the function at its origin, in the Infusorian, where I it is reduced to the mere impressionability (almost purely I chemical) of a pigment-spot to light: this function, pos- sibly only an accidental fact in the beginning, may have brought about a slight complication of the organ, which again induced an improvement of the function. It may have done this either directly, through some unknown mechanism, or indirectly, merely through the effect of the advantages it brought to the living being and the hold it thus offered to natural selection. Thus the progressive formation of an eye as well contrived as ours would be explained by an almost infinite number of actions and re- actions between the function and the organ, without the intervention of other than mechanical causes. The question ia hard to decide, indeed, when put di- I FauI Jaoet, Let Cautea finales, Paris, L876, p. 83. • Ibid. p. 80.
[OHAf.'
rectly between the function and the organ, as is done in the doctrine of finality, as also mechanism itself does. For organ and function are terms of different nature, and each conditions the other so closely that it is impossible to say a priori whether in expressing their relation we should l>egin with the first, as does mechanism, or with the second, as finalism requires. But the discussion would take an entirely different turn, we thinit, if we began by comparing together two terms of the same nature, an organ with an organ, instead of an organ with its function. In this case, it would be possible to proceed little by little to a solution more and more plausible, and there would be the more chance of a successful issue the more resolutely we assumed the evolutionist hypothesis.
Let us place side by side the eye of a vertebrate and that of a mollusc such as the common Pecten. We find the same essential parts in each, composed of analogous elements. The eye of the Pecten presents a retina, a cornea, a lens of cellular structure like our own. There is even that peculiar inversion of retinal elements which is not met with, in general, in the retina of the inverte- brates. Now, the origin of molluscs may be a debated question, but, whatever opinion we hold, all are agreed that molluscs and vertebrates separated from their common parent-stem long before the appearance of an eye so com- plex as that of the Pecten. Whence, then, the structural analogy?
Let us question on this point the two opposed systems
of evolutionist explanation in turn — the hypothesis of
purely accidental vanations, and that of a variation di-
rected in a definite way under the influence of external
conditions.
The first, as is well known, is presented to-day in two quite different forms. Darwin spoke of very slight van-
I-l
The Choice of an Example
ationa being accumulated by natural selection. He waa not ignorant of the facts of sudden variation ; but he thought these "sports," as he called thenn, were only monstrosities incapable of perpetuating themselves; and he accounted for the genesis of species by an accumulation of insensible variations.' Such is still the opinion of many naturaUsts. It is tending, however, to give way to the opposite idea that a new species comes into being all at once by the simultaneous appearance of several new characters, all somewhat different from the previous ones. This latter hypothesis, already proposed by various authors, notably by Bateson in a remarkable book," has become deeply Bignificant and acquired great force since the striking ex- periments of Hugo de Vries. This botanist, working on the (Enotkera Lamarckiana, obtained at the end of a few generations a certain number of new species. The theory he deduces from his experiments is of the highest interest. Species pass through alternate periods of stability and transformation. When the period of "mutabihty" occurs, unexpected forms spring forth in a great number of diflfer- ent directions." — We will not attempt to take sides be- tween this hypothesis and that of insensible variations. Indeed, perhaps both are partly true. We wish merely to point out that if the variations invoked are accidental, they do not, whether small or great, account for a similar- ity of structure such as we have cited.
Let us assume, to begin with, the Darwinian theory of
insensible variations, and suppose the occurrence of small
differences due to chance, and continually accumulating.
Darwin, Origin of Speciea, chap. ii.
• Baleaon, MatmaU for the Study oj Variation, Loodoii, 1894, e»- peciaUy pp. 567 ff. Cf. Scott, "Variationfl and Mutatjons" (.4tnertaui Journal of Scie7ice,Jio\. 1894).
* De Vries, Die Mvtatiojtttkeorvr, Leipzig, 1001-1903. Of., by the Mine author, Spedet and Variefiea, Clucago, 1905.
64 Creative Evolution
It must not be forgotten that all the parts of an organism are necessarily coordinated. Whether the function be the effect of the organ or its cause, it mattere little; one point is certain — the organ will be of no use and will not give selection a hold unless it functions. However the minute structure of the retina may develop, and however complicated it may become, such progress, instead of favoring vision, will probably hinder it if the visual centres do not develop at the same time, as well as several parts of the visual organ itself. If the variations are accidental, how can they ever agree to arise in every part of the organ at the same time, in such way that the organ will con- tinue to perform its function? Darwin quite understood this; it is one of the reasons why he regarded variation as insensible.' For a difference which arises accidentally at one point of the visual apparatus, if it be very slight, will not hinder the functioning of the organ; and hence this first accidental variation can, in a sense, watt for comple- mentary variations to accumulate and raise vision to a higher degree of perfection. Granted; but while the insensible variation does not hinder the functioning of the eye, neither does it help it, so long as the variations that are complementary do not occur. How, in that case, can the variation be retained by natural selection? Un- wittingly one will reason as if the slight variation were a toothing stone set up by the organism and reserved for a later construction. This hypothesis, so little conformable to the Darwinian principle, is difficult enough to avoid even in the case of an organ which has been developed along one single main line of evolution, e.g. the vertebrate eye. But it is absolutely forced upon us when we observe the likeness of structure of the vertebrate eye and that of the molluscs. How could the same small variations, incal- I Darwin, Origin o/ Spteia, chap. *
Insensible Variation
culable in number, have ever occurred in the same order
on two independent Unes of evolution, if they were purely
accidental? And how could they have been preserved
by selection and accumulated in both cases, the same in
the same order, when each of them, taken separately,
was of no use?