The Physical Basis of Mind. George Henry Lewes
atra, which lives high up among the mountains, brings forth its young full-formed. This animal never lives in the water. Yet if we open a gravid female, we find tadpoles inside her with exquisitely feathered gills, and (as I have witnessed) these tadpoles “when from the mother’s womb untimely ripped,” if placed in water, swim about like the tadpoles of water newts. Obviously this aquatic organization has no reference to the future life of the animal, nor has it any adaptation to its embryonic condition; it has solely reference to ancestral forms, it repeats a phase in the development of its progenitors. Again, in the embryo of the naked Nudibranch, we always observe a shell, although the animal is without a shell, and there can be no purpose served by the shell in embryonic life.47 Finally, the human embryo has a tail, which is of course utterly purposeless, and which, although to be explained as a result of organic laws, is on the creative hypothesis only explained as an adherence to the general plan of structure—a specimen of pedantic trifling “worthy of no intellect above the pongo’s.”48
105. Humanly appreciated, not only is it difficult to justify the successive stages of development, the incessant building up of structures immediately to be taken down, but also to explain why development was necessary at all. Why are not plants and animals formed at once, as Eve was mythically affirmed to be taken from Adam’s rib, and Minerva from Jupiter’s head? The theory of Evolution answers this question very simply; the theory of Creation can only answer it by affirming that such was the ordained plan. But the theory of Evolution not only gives the simpler and more intelligible answer to this question, it gives an answer to the further question which leaves the theory of Creation no loophole except a sophism—namely, why the formation of organisms is constantly being frustrated or perverted? And, further, it gives an explanation of the law noticed by Milne Edwards, that Nature is as economical in her means as she is prodigal in her variation of them: “On dirait qu’avant de recourir à des ressources nouvelles elle a voulu épuiser, en quelque sorte, chacun des procédés qu’elle avait mis en jeu.”49 The applause bestowed on Nature for being economical is a curious transference to Nature of human necessities. Why, with a whole universe at her disposal, should Nature be economical? Why must she always be working in the same groove, and using but a few out of the many substances at her command? Economy is a virtue only in the poor. If Nature, in organic evolutions, is restricted to a very few substances, and a very few modes of combination, always creating new forms by modification of the old, and apparently incapable of creating an organism at once, this must imply an inherent necessity which is very unlike the free choice that can render economy a merit.
106. There may indeed be raised an objection to the Development Hypothesis on the ground that if the complex forms were all developed from the simpler forms, we ought to trace the identities through all their stages. If the fish developed into the reptile, the reptile into the bird, and the bird into the mammal (which I, for one, think questionable), we ought to find, it is urged, evidence of this passage. And at one time it was asserted that the evidence existed; but this has been disproved, and on the disproof the opponents of Evolution take their stand. Although I cannot feel much confidence in the idea of such a passage from Type to Type, and although the passage, if ever it occurred, must have occurred at so remote a period as to leave no evidence more positive than inference, I cannot but think the teaching of Embryology far more favorable to it than to our opponents. Supposing, for the sake of argument, that the passage did take place, ought we to find the embryonic stages accurately reproducing the permanent forms of lower types? Von Baer thinks we ought; and lesser men may follow him without reproach. But it seems to me that he starts from an inadmissible assumption, namely, that the development must necessarily be in a straight line rather than in a multiplicity of divergent lines. “When we find the embryonic condition,” he says, “differing from the adult, we ought to find a corresponding condition somewhere in the lower animals.”50 Not necessarily. We know that the mental development of a civilized man passes through the stages which the race passed through in the course of its long history, and the psychology of the child reproduces the psychology of the savage. But as this development takes place under conditions in many respects different, and as certain phases are hurried over, we do not expect to find a complete parallel. It is enough if we can trace general resemblances. Von Baer adds, “That certain correspondences should occur between the embryonic states of some animals and the adult states of others seems inevitable and of no significance(?). They could not fail, since the embryos lie within the animal sphere, and the variations of which the animal body is capable are determined for each type by the internal connection and mutual reaction of its organs, so that particular repetitions are inevitable.” A profound remark, to which I shall hereafter have occasion to return, but its bearing on the present question is inconclusive. The fact that the embryonic stages of the higher animals resemble in general characters the permanent stages of the lower animals, and very closely resemble the embryonic stages of those animals, is all that the Development Hypothesis requires. Nor is its value lessened by the fact that many of the details and intermediate stages seem passed over in the development of the higher forms, for the recapitulation can only be of outlines, not of details; since there are differences in the forms, there must be differences in their histories.
107. In the preceding observations the object has simply been to show that the phenomena to be explained can be rationally conceived as resulting from gradual Evolution, whereas they cannot be so rationally interpreted on any other hypothesis. And here it may be needful to say a word respecting Epigenesis.
The Preformation hypothesis, which regarded every organism as a simple educt and not the product of a germ, was called by its advocates an evolution hypothesis—meaning that the adult form was an outgrowth of the germ, the miniature magnified. Wolff, who replaced that conception by a truer one, called his, by contrast, Epigenesis, meaning that there was not simply out-growth but new growth. “The various parts,” he says, “arise one after the other, so that always one is secreted from (excernirt), or deposited (deponirt) on the other; and then it is either a free and independent part, or is only fixed to that which gave it existence, or else is contained within it. So that every part is the effect of a pre-existing part, and in turn the cause of a succeeding part.”51 The last sentence expresses the conception of Epigenesis which embryologists now adopt; and having said this, we may admit that Wolff, in combating the error of preformation, replacing it with the truer notion of gradual and successive formation, was occasionally open to the criticism made by Von Baer, that he missed the true sense of Evolution, since the new parts are not added on to the old parts as new formations, but evolved from them as transformations. “The word Evolution, therefore, seems to me more descriptive of the process than Epigenesis. It is true that the organism is not preformed, but the course of its development is precisely the course which its parents formerly passed through. Thus it is the Invisible—the course of development—which is predetermined.”52 When the word Epigenesis is used, therefore, the reader will understand it to signify that necessary succession which determines the existence of new forms. Just as the formation of chalk is not the indifferent product of any combination of its elements, carbon, oxygen, and calcium, but is the product of only one series of combinations, an evolution through necessary successions, the carbon uniting with oxygen to form carbonic acid, and this combining with the oxide of calcium to form chalk, so likewise the formation of a muscle, a bone, a limb, or a joint has its successive stages, each of which is necessary, none of which can be transposed. The formation of bone is peculiarly instructive, because the large proportion of inorganic matter in its substance, and seemingly deposited in the organic tissue, would lead one to suppose that it was almost an accidental formation, which might take place anywhere; yet although what is called connective tissue will ossify under certain conditions, true bone is the product of a very peculiar modification, which almost always needs to be preceded by cartilage. That the formation of bone has its special history may be seen in the fact that it is the last to appear in the animal series, many highly organized fishes being without it, and all the other systems appearing before it in the development of the embryo. Thus although the mother’s blood furnishes all the requisite material, the fœtus is incapable of assimilating this material and of forming bone, until its own development has reached a certain stage. Moreover, when ossification does begin, it generally begins in the skull (in man in the clavicle); and the only approach to an internal skeleton in the Invertebrates is the so-called skull of the Cephalopoda. Not only is bone a late development, but