Page - 221 - in The Origin of Species

THEORY OF NATURAL SELECTION 221 of natural selection, can a variety live side by side with the parent species? If both have become fitted for slightly dif- ferent habits of life or conditions, they might live together; and if we lay on one side polymoi-phic species, in which the variability seems to be of a peculiar nature, and all mere temporary variations, such as size, albinism, &c., the more permanent varieties are generally found, as far as I can discover, inhabiting distinct stations,—such as high land or low land, dry or moist districts. Moreover, in the case of animals which wander much alx)Ut and cross freely, their varieties seem to be generally confined to distinct regions. Bronn also insists that distinct species never differ from each other in single characters, but in many parts ; and he asks, how it always comes that many parts of the organisa- tion should have been modified at the same time through variation and natural selection? But there is no necessity for supposing that all the parts of any being have been simultaneously modified. The most striking modifications, excellently adapted for some purpose, might, as was formerly remarked, be acquired by successive variations, if slight, first in one part and then in another; and as they would be transmitted all together, they would appear to us as if they had been simultaneously developed. The best answer, how- ever, to the above objection is afforded by those domestic races which have been modified, chiefly through man's power of selection, for some special purpose. Look at the race and dray horse, or at the grey-hound and mastiff. Their whole frames and even their mental characteristics have been modified; but if we could trace each step in the history of their transformation,—and the latter steps can be traced,— we should not see great and simultaneous changes, but first one part and then another slightly modified and improved. Even when selection has been applied by man to some one character alone,—of which our cultivated plants offer the best instances,—it will invariably be found that although this one part, whether it be the flower, fruit, or leaves, has been greatly changed, almost all the other parts have been slightly modified. This may be attributed partly to the prin- ciple of correlated growth, and partly to so-called spon- taneous variation.