In the preceding article we directly intimated, on several occasions, that no theory other than that of reversion can afford a solution of the mystery of the appearance of favorable modifications. As some little diversity of opinion exists respecting Darwin's views on the subject of the cause of variation, it may be well for us to dwell awhile on this question, and to furnish some evidence substantiating our statement.
Darwin, in his Origin of Species, candidly and frankly admits that he can assign no satisfactory reason for the appearance of favorable modifications. He ascribes them to "spontaneous variability," and assures us that "our ignorance of the laws of variation is profound." We might adduce a number of other expressions equally declaratory of his inability to assign the cause of variation; but as the Duke of Argyll has taken such pains to direct attention to this hiatus in Darwin's evidence, we cannot refrain from quoting from his The Reign of Law:
"It has not, I think, been sufficiently observed that the theory of Mr. Darwin does not address itself to the same question, (the introduction of new forms of life,) and does not even profess to trace the origin of new forms to any definite law. His theory gives an explanation, not of the processes by which new forms first appear, but only of the processes by which, when they have appeared, they acquire a preference over others, and thus become established in the world. A new species is, indeed, according to his theory, as well as with the older theories of development, simply an unusual birth. The bond of connection between allied specific and generic forms is, in his view, simply the bond of inheritance. But Mr. Darwin does not pretend to have discovered any law or rule according to which new forms have been born from old forms. He does not hold that outward conditions, however changed, are sufficient to account for them. Still less does he connect them with the effort or aspirations of any organisms after new faculties and powers. He frankly confesses that 'our ignorance of the laws of variation is profound;' and says that in speaking of them as due to chance, he means only 'to acknowledge plainly our ignorance of the cause of each particular variation.' Again he says, 'I believe in no law of necessary development.'" (P. 228.)
On page 254, the Duke of Argyll continues:
"It will be seen, then, that the principle of Natural Selection has no bearing whatever on the origin of species, but only on the preservation and distribution of species when they have arisen. I have already pointed out that Mr. Darwin does not always keep this distinction clearly in view; because he speaks of natural selection 'producing' organs or 'adapting' them. It cannot be too often repeated that natural selection can produce nothing whatever except the conservation or preservation of some variation otherwise originated. The true origin of species does not consist in the adjustments which help varieties to live and prevail; but in those previous adjustments which cause those varieties to be born at all. Now, what are these? Can they be traced or even guessed at? Mr. Darwin has a whole chapter on the laws of variation, and it is here, if anywhere, that we look for any suggestion as to the physical causes which account for the origin as distinguished from the preservation of the species. He candidly admits that his doctrine of natural selection takes cognizance of variations only after they have arisen, and that it regards variations as purely accidental in their origin, or, in other words, as due to chance. This, of course, he adds, is a supposition wholly incorrect, and only serves 'to indicate plainly our ignorance of the cause of each particular variation.' Accordingly, the laws of variation which he proceeds to indicate are merely certain observed facts in respect to variation, and do not at all come under the category of laws, in that higher sense in which the word law indicates a discovered method under which natural forces are made to work."
It will be seen that we have not gone too far in proclaiming Darwin's inability to account for variation. In the absence, then, of any other rational explanation, are we not necessitated to accept the theory of reversion? What possible objection can be urged against it? Reversion is not a heretofore unknown factor. Nor is it an occult factor. It is constantly recognized by Darwin. Two chapters of the Animals and Plants under Domestication are filled with phenomena illustrating its action; and it forms the basis of his lately propounded hypothesis of pangenesis.
In the interval between the publication of his Origin of Species and the writing of his Animals and Plants under Domestication, Darwin has received no enlightenment as to the cause of variation. A writer in The North American Review for October, 1868, holds the contrary, and distinctly asserts that Darwin is inclined to adopt the mechanist theory, to attribute the phenomena of variation solely to the influence of the physical conditions, and to repudiate the idea of a concurrent cause. After speaking of Mr. Herbert Spencer's ascription of variations to the physical conditions, he says:
"In his latest work, Mr. Darwin inclines to adopt the mechanist theory, so far as the cause of variations is concerned. 'We will now consider,' he says, 'the general arguments, which appear to me to have great weight, in favor of the view that variations are directly or indirectly caused by the conditions of life to which each being, and more especially its ancestors, have been exposed.... These several considerations alone render it probable that variation of every kind is directly or indirectly caused by changed conditions of life. Or, to put the case under another point of view; if it were possible to expose all the individuals of a species to absolutely uniform conditions, there would be no variability.' When variations of all kinds and degrees, that is, all the gradual differentiations by which the vast multitude of existing species has been evolved out of the primordial form or forms, are thus attributed solely to the accumulative action of the conditions of life, without any recognition of a concurrent cause in that constant self-adaptation by organisms for which the conditions cannot account, it would seem fairly inferrible that the mechanist theory is supposed to explain the evolution of the species, if not of individual organisms."
Now, there is nothing in the expressions quoted from Darwin's work, which justifies such a construction as The North American Review has here placed upon them. Although we, as a vitalist, implicitly believe in the coöperation of other than mechanical causes, yet we fully and most unqualifiedly concur in Darwin's assertion that there would be no variability were all the individuals of a species exposed to absolutely uniform conditions. This fact is by no means incompatible with a belief in "forces which manifest themselves in the organism." We have shown that varieties or races under nature are attributable solely to the action of the conditions of life. Under domestication, the changed conditions are the secondary cause of favorable modifications, reversion being the primary cause. But without the concurrence of this secondary cause, it is wholly impossible for favorable variations to occur. The expressions of Darwin, then, carry with them no implication that variations are solely caused by the changed condition; for the recognition of the power of the conditions to the extent claimed by Darwin by no means precludes the belief in a concurrent cause. The conclusion that a change in the conditions is a cause of variation, and that were there no such change there would be no variability, is necessitated by the theory here advanced. For, an acquaintance with phenomena displaying the action of the physical conditions forces upon us the teleological inference that certain conditions are essential to the full development of characters. Does it not thence necessarily follow that, when the conditions are dissimilar, modifications will result from the individuals of a species being exposed to conditions favorable or unfavorable in different degrees to the growth of some of the parts or features? Darwin's assertion is then quite consistent with a belief in the concurrence of causes not mechanical.
But the discovery of Darwin's opinion on this point is not left solely to conjecture and speculation. Had the North American Reviewer carefully perused Darwin's late work, he would have found many most unequivocal declarations of the author's belief in the concurrence of other causes. They recur most frequently.