Chapter V: Evolution (3)

137. The application of this illustration is plain. Just as a wind-instrument is incapable of becoming a stringed instrument, so a Mollusc, with all its muscles unstriped, and its nervous system unsymmetrical, is incapable of becoming a Crustacean, with all its muscles striped and its nervous system symmetrical. Indeed there are probably few biologists of the present day who imagine the transmutation of one kind into the other to be possible; but many biologists assume that both may have been evolved from a common root. The point is beyond proof; yet I think there is a greater probability in the assumption that both were evolved from different roots. At any rate, one thing is certain; a divergence could only have been effected by a series of _substitutions_; and the question when and how these substitutions took place is unanswerable: one school believes them to have been creative fiats, the other school believes them to have been transmutations.

138. When we see an annelid and a vertebrate resembling each other in some special point which is not common either to their classes or to any intermediate classes--as when we see the wood-louse (_Oniscus_) and the hedgehog defend themselves in the same strange way by rolling up into a ball--we cannot interpret this as a trace of distant kinship. When we see a breed of pigeons and a breed of canaries turning somersaults, and one of the Bear family (_Ratel_) given to the same singular habit, we can hardly suppose that this is in each case inherited from a common progenitor. When we see one savage race tipping arrows with iron, and another, ignorant of iron, using poison, there is a community of object effected by diversity of means; but the analogy does not necessarily imply any closer connection between the two races than the fact that men with similar faculties and similar wants find out similar methods of supplying their wants. Even those who admit that the human race is one family, and that the various peoples carried with them a common fund of knowledge when they separated from the parent stock, may still point to a variety of new inventions and new social developments which occurred quite independently of each other, yet are strikingly alike. Their resemblance will be due to resemblance in the conditions. The existence, for example, of a religious worship, or a social institution, in two nations widely separated both in time and space, and under great historical diversities, is no absolute proof that these two nations are from the same stock, and that the ideas have the same parentage. It may be so; it may be otherwise. It may be an analogy no more implying kinship than the fact of ants making slaves of other ants (and these the black ants!) implies a kinship with men. Given an organization which in the two nations is alike, and a history which is in certain characteristics analogous, there must inevitably result religious and social institutions having a corresponding resemblance. I do not wish to imply that the researches of philologists and ethnologists are misdirected, or that their conclusions respecting the kinship of mankind are to be rejected; I only urge the consideration that perhaps too much stress is laid on community of blood, and not enough on community of conditions.

RECAPITULATION.

139. The various lines of argument may here be recapitulated. The organic world presents a spectacle of endless diversity, accompanied by a pervading uniformity. The general resemblances in forms and functions are more or less masked by particular differences. The resemblances, it is said, may be all due to kinship, all the living individuals having descended from a primordial cell; and at each stage of the descent the adaptations to new conditions may have issued in deviations from the ancestral form, while the process of Natural Selection giving stability to those variations which best fitted the organism in the struggle of existence, has made greater and greater gaps, and produced more marked diversities among the descendants. This is the Darwinian Theory: “On my theory unity of Type is explained by unity of Descent.”

140. By the general consent of biologists, this theory is held to explain many if not all the observed facts. It is a very luminous suggestion; but it requires an enlargement so as to include Organic Affinity; and when once this fundamental principle is admitted, it brings with it very serious doubts as to the theory of Descent. We are then entitled to assume that many of the most striking resemblances, instead of being due to kinship, are due simply to the general principle that similar causes must have similar effects, and that organic substances having a very close resemblance, organized substances must have similar stages of evolution under similar conditions; and thus organs will necessarily take on very similar forms in very different organisms (for example, the eye of the cephalopod and the eye of the vertebrate), and organisms having widely different parentage may closely resemble each other. If we are entitled to assume that protoplasm appeared not in one microscopic spot alone, but in many places and in vast quantities--and this is surely the more justifiable assumption--then we must also admit that these germinal starting-points were from the first, or very shortly afterwards, differentiated by variations in their elementary composition. Now we know that a very minute change in composition may lead to immense differences in evolution. Thus the descendants of two slightly different progenitors may, by continual differentiation, become very markedly unlike; yet, because of the original resemblance of their substances, they will reveal a pervading similarity.

While it is thus conceivable that all organisms may resemble each other, and all differ, owing to the similarities and diversities in the “conditions of existence” (and among those conditions that of descent is of wide range), it is not very readily conceivable how advantage in the external struggle could have determined the varieties of form and function, because many differentiations give no superiority in the struggle. As Mr. St. George Mivart urges, “Natural Selection utterly fails to account for the conservation and development of the minute and rudimentary beginnings, the slight and infinitesimal commencements of structures, however useful those structures may afterwards become.”[78] And this is undeniable on the supposition that Natural Selection is an agency not identical with the variations of growth, but exclusively confined to the accumulation of favorable variations.

141. In estimating the two hypotheses--First, of Descent from one primordial germ, and the modifications _due_ to Natural Selection, or, as I should say, _expressed_ in Selection; and Secondly, of Descent from innumerable germs having initial differences, which differences radiated into the marked modifications, there is this superiority to be claimed for the first, that it is more easily handled as an aid to research, and is therefore more decidedly useful. The laws of Organic Affinity are at present too obscure for any successful application. I only wish to point out that the theory of Descent is an imaginary construction of what _may have been_ the process of species-formation, not a transcription of the process observed. It constructs an imaginary Type as progenitor of a long line of widely different descendants. The annelid which is taken as the ancestor of the vertebrates is not any annelid known either to zoölogists or geologists, but a generalized and imaginary type. So daringly liberal is the imagination in endowing the ancestor with whatever may be required for the descendants, that Mr. Darwin thinks it _probable_, from what we know of the embryos of vertebrates, that these animals “are the modified descendants of some ancient progenitor which was furnished in its adult state with branchiæ, a swim-bladder, four simple limbs, and a long tail, all fitted for an organic life,” (p. 533); and Dr. Dohrn conceives the original type to have contained within itself all that has been subsequently evolved in the highest vertebrate, the other and less elaborate organisms being mere degradations from this type.[79] This use of the imagination, although not without advantages, is also not without dangers. It may direct research, it must not be suffered to replace research.

PROBLEM II.

THE NERVOUS MECHANISM.

“All the functions of the nervous system are as dependent upon its
structure and nature, as the accurate indication of time upon the
construction of the chronometer.”--PROCHASKA.

“Unser Wissen wird nie vollendet, ist und bleibt Stückwerk; dessen
Ergänzung das Streben und Hoffen der forschenden Denker bleiben
wird für alle Zeit.”--RADENHAUSEN, _Osiris_.

“Our nimble souls
Can spin an insubstantial universe
Suiting our mood, and call it possible,
Sooner than see one grain with eye exact,
And give strict record of it.”
GEORGE ELIOT, _The Spanish Gypsy_.

“If we compare the teachings of our books with what Nature is
constantly showing, we find there is no agreement between those two
sources of learning.”--BROWN SÉQUARD.

THE NERVOUS MECHANISM.