Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/207

Rh same time by retrograding metamorphoses, as seen in parasites, etc.

While Natural Selection is generally admitted to be a sufficient cause for the production of unimportant variations, it is often objected that important structures, such as the skeleton, could never be modified by such a process. Mr. Darwin, however, has shown that the skeleton is as susceptible to modification as any other part of the organization. Thus, the different kinds of pigeons, supposed unanimously by " fanciers" to have descended from different ancestors, but which are now known to be the posterity of the Rock Pigeon, offer great variations in their skeleton, as in the number of their vertebrae and ribs, in the character of the breast-bone, merry-thought, lower jaw, and bones of the face. All zoologists admit that the various kinds of rabbits have descended from a common stock; and yet the greatest difference is seen in the size, shape, and form of the skull, in the character of the backbone, etc. But not only have the changing of conditions and the domestication of animals modified the skeleton, which is regarded by anatomists as one of the most constant of characters, but the viscera and all other parts of the organization have been affected. We do not regard, therefore, the objection of Natural Selection not being a sufficient cause of change as of any weight. The fact of Hybrids often not breeding is regarded by many as an important objection. The case of the mule not breeding is usually referred to. This objection does not seem to us to amount to much, as it is well known that the Porto Santo rabbit, which is the offspring of the European rabbits placed on that island in 1419, will not breed now with the posterity of its European ancestor. Further, it does not follow, because mules are unreproductive, that all other hybrids have been, and will be. Thus, the Lepus Darwinii, originally resulting from the crossing of the Rabbit and Hare, now reproduces its kind, the animal