v.] IN THE THEORY OF NATURAL SELECTION. 267 



body of an individual or of any of its ancestors, not even the 

 minutest and most insignificant part, which has arisen in any 

 other way than under the influence of the conditions of life ; 

 and the parts of the body conform to these conditions, as the 

 channel of a river is shaped by the stream which flows over it. 



These are indeed only convictions, not real proofs ; for we 

 are not yet sufficiently intimately acquainted with any species 

 to be able to recognize the nature and meaning of all the details 

 of its structure, in all their relations : and we are still less able 

 to trace the ancestral history in each case, and to make out the 

 origin of those structures of which the presence in the de- 

 scendants depends primarily upon heredity. But already a fair 

 advance towards the attainment of inductive proof has been 

 made ; for the number of adaptations which have been estab- 

 lished is now very large and 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 re- 

 mains 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 dis- 

 tinguishes 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 external 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 pecu- 



