IN THE THEORY OF NATURAL SELECTION. 261 



is increasing every day. If, however, we anticipate the results of 

 future researches, and admit that an organism only consists of 

 adaptations, based upon an ancestral constitution, it is obvious 

 that nothing remains to be explained by a phyletic force, even 

 though the latter be presented to us in the refined form of Nageli's 

 self-changing idioplasm. 



It will perhaps be useful to illustrate my views by a familiar 

 example. I choose the well-known group of the whales. These 

 animals are placental mammals, which, probably in secondary times, 

 arose from terrestrial Mammalia, by adaptation to an aquatic life. 



Everything that is characteristic of these animals and distin- 

 guishes them from other mammals depends upon this adaptation. 

 Their fore-limbs have been transformed into rigid paddles, only 

 movable at the shoulder-joint ; upon the back and the tail there 

 are ridges with a form somewhat similar to the dorsal and caudal 

 fins of fishes. The organ of hearing is without any external 

 ear and without an air-containing external auditory meatus. The 

 aerial vibrations do not pass, as in other mammals, from the ex- 

 ternal auditory passage to the tympanic cavity and thus to the 

 nerve-terminations of the inner ear ; but they reach the tympanic 

 cavity by direct transmission through the bones of the skull, 

 which possess a special structure and contain abundant air-cavities. 

 This arrangement is obviously adapted for hearing in water. The 

 nostrils also exhibit peculiarities, for they do not open near the 

 mouth, but upon the forehead, so that the animal can breathe, 

 even in a rough sea, as soon as it comes to the surface. In 

 order to facilitate rapid movement in water, the whole body has 

 become extended in length, and spindle-shaped, like the body of 

 a fish. The hind limbs are absent in no other mammals, the fish- 

 like Sirenia being alone excepted. In the whales, as in the Sirenia, 

 these appendages have become useless, owing to the powerfully 

 developed tail-fin ; they are now rudimentary and consist of some 

 small bones and muscles deeply buried in the body of the animal, 

 which nevertheless, in certain species, still exhibit the original 

 structure of the hind-limb. The hairy covering of other mam- 

 mals has also disappeared, its place having been taken by a thick 

 layer of fat beneath the skin, which affords a much better pro- 

 tection against cold. This fatty layer was also necessary in order 

 to diminish the specific gravity of the animal, and to thus render 



