Before treating of variation under domestication, we may take occasion to disclaim any attempt to account for variations of color. These are not so manifestly due to degeneration and subsequent favorable reversion. They accord with our theory; but as this accordance is not susceptible of the short and complete demonstration of that of all other variations, the limits of our series preclude our entering into a long dissertation on the subject. Nor would the importance of modifications of color justify such a course; for Darwin characterizes them as phenomena of no consequence, and assures us that little attention is paid to them by naturalists.
Under domestication, animals and plants are subjected to comparatively favorable conditions, to conditions of which they have been deprived in the state of nature. Thus stimulated, they display marked improvement, and revert to the perfect condition from which they have degenerated. The favorable changes which they present are noted by man, and carefully preserved by crossing and judicious pairing with those possessing equal advantages. In this way, the best are selected and made to transmit to their offspring their improved condition. Each breeder's success is determined by the more or less favorable conditions of the situation, district, or country, and by his sagacity and discrimination in selecting those in which occurs the greatest increase of size. As the conditions vary in different localities, and as breeders possess different degrees of scientific knowledge, animals and plants would be differently improved, and thus there is established a series of gradations all answering to the characters of as many varieties. As we have seen, in a somewhat similar manner races have been formed under nature. They were in part established by the retention of the animal or plant in several of the phases of degeneration; while varieties under domestication are in part due to the retention of the organism at each stage of reversion. The greater number of varieties under domestication, as compared with the paucity of races under nature, results in a measure from man's selection retaining the organism at almost every gradation. Under nature, the animals of a district or country freely intercross, and from this intercrossing results uniformity of character and the consequent existence of only one race in a country. Besides, the conditions of life are comparatively uniform in each district; but under domestication man is, by means of his scientific knowledge, continually varying the conditions.
We are conscious that this explanation accounts only for difference of size. It does not show how wholly different characters have been acquired by the various varieties; nor the cause of the possession of the greatest structural differences by individuals of the same species. Were this the sole process by which varieties were formed, one variety would be merely the miniature of the other. Other explanations are required to illustrate the manner in which the great divergence of character observable under domestication, has been effected. These we shall furnish.
Darwin, both in his Origin of Species and in his Animals and Plants under Domestication, draws particular attention to this divergence of character. It forms a most conspicuous portion of his theory. It displays the gradual acquisition by individuals originally alike of differences as great as those characterizing species.
As Darwin has assured us, there is scarcely a single species under nature which does not possess organs in a rudimentary state. Now, these arise under domestication, and are apportioned among the several varieties. Each organ is developed, and is allotted to a certain variety, of which it forms the peculiarity. In one variety, special attention is paid to the development of a single organ, while the remaining organs are left to be developed in and to form the characteristics of other varieties. Thus the upwardly-expanded tail in the pigeon constitutes the peculiarity characteristic of the fantail; the enlargement of the œsophagus, that of the pouter; and the divergent feathers along the front of the neck and breast, that of the turbit.
By this process—the development of rudimentary organs and their apportionment among the several varieties—a portion of the divergence of character is effected.
These rudimentary organs have been the occasion of many a warm controversy. They are asserted to be totally incongruous with the doctrine of teleology. Their uselessness and occasionally detrimental nature, it is contended, preclude the possibility of design. Several objections have been urged against the doctrine of final causes; but those who profess to disbelieve in design concur in according to these organs the greatest prominence.
The doctrine of final causes is a conception thrust upon us by a vast multitude of facts from organic nature. But, now and then, exceptional phenomena will present themselves apparently at variance with it. These, as a writer in The London Quarterly Review for July, 1869, ably maintains, are merely objections, not disproofs. Owing to a misconception current among the advocates of special creation, they have been unable to reconcile rudimentary organs with the doctrine of teleology. All the attempts heretofore made to harmonize these anomalous features with the doctrine of final causes have been feeble. We may instance one. A Mr. Paget, in his Hunterian Lectures at the College of Surgeons, argues that the function of these organs is "to withdraw from the blood some elements of nutrition, which, if retained in it, would be positively injurious." We can readily appreciate the feelings which induce an evolutionist to smile at this assumption of excretion as the sole function and purpose of a rudimentary organ.
Upon the theory of degeneration and subsequent favorable reversion here propounded, these rudimentary organs are quite congruous with the doctrine of final cause. To obviate the difficulty presented by these parts, we have accepted the interpretation of the evolutionist. This interpretation we adopted at the start. It forms the basis of our theory—its foundation-stone. That for which the evolutionist contends is, that these organs have at one period been fully developed. In this we concurred; for it furnished us with an explanation of the favorable modifications under domestication; while, as we shall show, it is by no means at variance with the doctrine of the immutability of the species. Rudimentary organs imply degeneration, past complexity of structure, and present comparative simplicity of structure; facts at variance with evolution, but strictly in accordance with our theory. We have seen that the idea of the normal nature of the existing natural condition has rendered the advocates of special creation unable to account for the appearance of profitable modifications. The seeming incongruity between rudimentary organs and the doctrine of teleology is a result of the same misconception. A curious confusion of ideas, generated by the assumption of this false position, has urged the opponents of evolution tacitly to contend that animals and plants were originally created with these organs in a rudimentary state, and that the present condition of these parts is a normal one. We, concurrently with the evolutionists, recognize in these organs "traces of old laws"—"records of the past." They are the traces of laws which obtained when the conditions were favorable to the full development of the organs. Under domestication, the conditions are being supplied, and the organs are, in consequence, being developed. On page 386 of his Principles of Biology, Mr. Herbert Spencer says, "And then to complete the proof that these undeveloped parts are marks of descent from races in which they were developed, there are not a few direct experiences of this relation. 'We have plenty of cases of rudimentary organs in our domestic productions—as the stump of a tail in tailless breeds—the vestige of an ear in ear-less breeds—the reappearance of minute dangling horns in hornless breeds of cattle.'"
But together with their being traces of old laws, they are traces of laws which so far adhere to the present that the laws of the whole organism fail fully to obtain without their concurrence; and their concurrence is consequent solely upon the full development of these rudimental features. In other words, full perfection consists in the perfect coördination of all the parts, and absence of this coördination suffices to throw the organism within the domain of pathology. The reduction, therefore, of any organ to a rudimentary condition is deleterious to the organism as a whole. We are perfectly aware that this needs something more than gratuitous affirmation; but as the adduction of evidence in this place would be inconsistent with the symmetry and continuity of our argument, we are forced to bespeak our readers' indulgence until the publication of the next article of this series. But it is sufficiently clear that, upon assuming the truth of our theory, the difficulty offered to the doctrine of final causes by rudimentary organs is obviated.