162 THE PROBLEMS OF EVOLUTION 



the ocean. ^^ His evidence is a masterly illustration 

 of the isolation principle, even though it goes no 

 further. And finally Schwarz's study of the Mona 

 monkeys is one of the finest expositions of the diver- 

 sification of animal species that I have read. He 

 shows the occurrence of subspecies separated by 

 natural barriers, the existence of varying degrees 

 of diversity among the several divisions recognized, 

 and in some cases even the overlapping of sub- 

 species which have become too different to inter- 

 breed, although they now occupy the same areas. ^^ 

 Rensch's work on geographic variation is an exten- 

 sive treatment of the subject which describes nu- 

 merous examples of a similar kind.^^ 



Unfortunately mere observation of natural con- 

 ditions does not show how the conditions arose. 

 Organs may be associated with environmental 

 conditions to which they are not due. Whether 

 mutation, natural selection, isolation, preadapta- 

 tion, or individual adaptation, has been the cause 

 of a given condition we must often be unable to 

 determine, although the few experiments cited and 

 the many observations on record suggest the co- 

 operation of all of these factors in bringing the 

 population of the world to its present state. Even 

 the close coincidence of graded series of variations 



^^ The Tanganyika Problem, 1903. 



1^ Schwarz, Zeitschr. f. indukt. Abst. u. Vererbungsl., 1928, Suppl. Bd. 11, 

 129^1399. 

 20 Op. cit. 



