OEIGIN OF THE SPECIFIC TYPE 333 



species which are subject to the same influence must undergo the 

 same variation. Transformations of this kind have exactly the 

 appearance of resulting from ' an internal evolutionary force,' such as 

 Nageli assumed, and the unity of the specific type will not be disturbed 

 by them. 



Nor will this occur, as far as I can see, if the transformation 

 of a species depends solely upon new adaptations and their internal 

 consequences, for if a particular organism has to adapt itself to special 

 new conditions, it will usually be able to do so only in one way, and 

 thus natural selection will always allow the same suitable variational 

 tendencies to survive and reproduce, so that the unity of the specific 

 type will not be permanently disturbed in this way either. The more 

 advantageous the new conditions of .life prove, and the more diverse 

 the ways in which they can be utilized, the more rapidly will the 

 species first adapted to them multiply, and the more will their 

 descendants be impelled to adapt themselves specially to the different 

 possibilities of utilizing the new situation, and thus, from a parent 

 species adapted in general to the new conditions, there arise forms 

 adapted to its more detailed possibilities. I must refer again to the 

 previous ■ instance of the Cetaceans which originated from vegetarian 

 littoral, or fluviatile mammals, and have evolved since the Triassic 

 period into a very considerable number of species-groups. All are 

 alike in their general adaptation, and these adaptations to the condi- 

 tions of life of aquatic animals — the fish-like form, the flippers, the 

 peculiarities of the respiratory organs and the organs of hearing — 

 when once acquired v/ould not and could not be lost again ; but each 

 of the modern groups of whales has its particular sphere of life, 

 which it effectively exploits by means of subordinate adaptations. 

 Thus there are the dolphins with their bill-like jaws and the two 

 rows of conical teeth, their active temperament, rapid movements, and 

 diet of fish; and the whalebone whales with their enormous gape, 

 the sieve-apparatus of whalebone-plates, and a diet of small molluscs 

 and the like. But each of these groups has split up into species, and 

 if we again regard the principle of adaptation as determining and 

 directing evolution, we are no nearer being able to prove the assump- 

 tion in regard to individual cases, for we know too little about the 

 conditions of life to be able to demonstrate that the peculiarities 

 of structure are actually adaptations to these. Theoretically, however, 

 it is quite easy to suppose that adaptation to a particular sphere of life 

 was the guiding factor in their evolution, and if this be so — as we have 

 already proved that it is in regard to the two chief groups and the 

 whole class — then the harmony of the structure must be due solely 



Digitized by Microsoft® 



