316 GENERAL BIOLOGY 



tion was attracted to the problem in the first place 

 by observing that in the various islands of the 

 Galapagos group, off the western coast of South 

 America, although numbers of genera of land animals 

 are to be found on each of the islands, as well as on 

 the mainland, yet each island has its own species, 

 differing slightly, but definitely, from those of ad- 

 jacent islands. There is little doubt but that at 

 one time the islands were all connected with one 

 another and with the mainland, and populated with 

 one species of rabbit or of grasshoppers or other 

 forms. When the islands came into existence through 

 the depression of the land, the consequent segrega- 

 tion and isolation of the different groups, and their 

 enforced inbreeding, called into being the new and 

 divergent types now to be found there. 



Such a segregation need not even be physical. A 

 mutual infertility may arise between groups of a 

 species in a common habitat which would just as 

 effectively segregate them, so far as reproducing 

 the species is concerned, as if a physical barrier were 

 erected. 



Darwin's hypothesis of Natural Selection is thus a 

 theory of the origin of species (evolution being 

 assumed) through the mutual relation of organisms 

 to their environment, such that the unadapted are 

 eliminated and a changing environment produces 

 changing types. Its most important feature is the 

 emphasis which it la'ys upon the passivity of the 

 organism itself. The selection is purely mechanical, 

 and the conclusions as regards the first two premises 



