« ПредыдущаяПродолжить »
but that the formation of species can take place only with the assistance of isolation has been effectively refuted by Weismann.48 He has shown that an "intercrossing of the incipient variety with the aboriginal form is not avoided by isolation;" and by the very favourable instance of the lake of Steinheim, among others, he has exhibited the formation of new species in the midst of the old ones. On Haeckel's remark that in the asexual propagation of the lower beings, the influence of intercrossing was not to be feared, Wagner had already restricted the necessity of isolation to the higher organisms with separate sexes. But Weismann most justly insists that Wagner's "law of migration" is deprived of all foundation by one of the most remarkable examples of the formation of varieties on the same territory, namely, the fact of the separation of the sexes, as to the derivation of which from species once hermaphrodite, all (the believers in Creation naturally excepted) are assuredly of one accord.
As we have already mentioned, it seems that if the impulse to form varieties once exists, the tendency spreads rapidly. Steinheim, with its Planorbis multiformis, is specially propitious to the demonstration of these periods of variation. If isolation coincides with such a period, it effects the establishment of new varieties into species without the aid of natural selection. As Darwin admits in his work on the origin of Man, he formerly bestowed too little attention on the formation of so-called morphological species. By this we mean, species not distinguished from their aboriginal stocks by any physiological advantages, and hence not superior to them, in which therefore the principle of
selection in the strict Darwinian sense is inapplicable. Two species of butterflies, differing only in a few specks or pencilings, or the notches on the wings, are in our estimation of perfectly equal physiological value; they are morphological species. Weismann sets up the proposition that "the colouring and penciling of the upper surface of the wing in butterflies are to be regarded as purely morphological characters, excepting in cases of mimicry and protective uniform colouring." He shows also by other examples that, "under certain circumstances and within a comparatively small range, new as well as morphological characters may be established by the effects of isolation only." The inapplicability of natural selection to the evolution of purely morphological variations was first pointed out by Nageli.49 With reference to this subject, Darwin with magnanimous modesty observes: "I now admit, after reading the essay by Nageli on plants, and the remarks by various authors with respect to animals, more especially those recently made by Professor Broca," that, in the earlier editions of my 'Origin of Species,' I probably attributed too much to the action of natural selection or the survival of the fittest. I have altered the fifth edition of the 'Origin' so as to confine my remarks to adaptive changes of structure. I had not formerly sufficiently considered the existence of many structures, which appear to be, as far as we can judge, neither beneficial nor injurious, and this I believe to be one of the greatest oversights as yet detected in my work." 51
We are disposed to think that the oversight with which Darwin charges himself is not so great, as it is here a question of the more indifferent species, not affect
ing the great phenomena of progressive development, and of which the origin is perfectly comprehensible by variability alone, or in any case, as we have seen, by variability with the co-operation of isolation. The value of natural selection is in no way deteriorated by the possibility of explaining the purely morphological species without its aid. In certain cases of mimicry, or the formation of natural protective masks and imitations, and for the explanation of organic beauty, natural selection seems inadequate. But what does this prove, but that, as all know, future generations must needs carry on the edifice? The additions which the presence of the theory of selection has been able to supply are scarcely worthy of mention.
As the type has become the family, and the system, as the shortest expression of the kindred relations of organisms, requires at the root of the genealogical tree a number of the lowest and simplest organisms, or perhaps one single primordial form, we must come to an understanding as to the problem of the beginning of life. Even quite recently, in March 1873, Max Müller, in accordance with an opinion shared by many, has again proclaimed "the Darwinian theory vulnerable at the beginning and at the end."52 Whether any considerable points of attack are offered by the final proposition of Darwinism, namely, the application of natural selection to man, and his sole characteristic peculiarity, language, we shall have another opportunity of inquiring. But what the renowned linguist terms the vulnerable beginning of Darwinism, the origin of life, has in fact nothing to do with actual Darwinism, or natural selection, unless the principle of selection be extended to the inorganic
world of matter. But the objection which endeavours to cut away the ground from under the doctrine of Descent, not the theory of selection, and represents the origin of life as incomprehensible and supernatural, we naturally regard as an attempt to gain a precedent for the supernatural creation of language. Between beginning and end, we naturalists may do as we please.
But it is strange that the very side which is so ready to reproach us with a want of philosophic method and induction, should here, where the material substratum is deficient, dispute the claims of the investigation of nature to its logical inferences. In the last page of the "Origin of Species," Darwin says: "There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been and are being evolved." In this concession, Darwin has certainly been untrue to himself; and it satisfies neither those who believe in the continuous work of creation by a personal God, nor the partizans of natural evolution. It is directly incompatible with the doctrine of Descent, or, as Zöllner 52 says: "The hypothesis of an act of creation (for the beginning of life) would not be a logical but a merely arbitrary limitation of the causal series against which our intellect rebels by reason of its inherent craving for causality. Whoever does not share this craving is beyond help, and he cannot be convinced. To hold the beginning of life as an arbitrary act of creation, is to break with the whole theory of cognition."
The verdict, as to the beginning of life, is commonly dependent on the standpoint adopted with respect to the possibility of primordial or spontaneous generation (generatio equivoca). This course is, in our opinion, only half correct. The subtlest experiments on spontaneous generation, whether from organic matter or from constituents not yet combined into molecules of organic matter, have proved indecisive on both sides. Neither the impossibility nor the possibility can be experimentally demonstrated; it always remains open to the sceptic to say, if nothing appears, that the failure of spontaneous generation is due to the conditions of the experiment; or if anything does make its appearance, that, notwithstanding every precaution, germs made their way into the infusion. Opinion as to continued primordial genesis still taking place, is thus a mere emanation of the general theory of nature held by each individual. To any one who holds open the possibility that, even now, animate may be evolved from inanimate existence, without the mediation of progenitors, the first origin of life in this natural method is at once self-evident. But even if the proof were given, which never can be given, that in the present world spontaneous generation does not occur, the inference would be false that it never did occur. When our planet had reached the phase of development in which the temperature of the surface admitted of the formation of water and the existence of albuminous substances, the quantitative and qualitative conditions of the atmosphere were different from what they now are. A thousand circumstances now beyond our control, and as to the possible nature of which it is needless to speculate, might lead to the pro- .