OTHER THEORIES OF SPECIES-FORMING. 251 



change in his stock than if he had left them all to breed 

 together ? 



"Well," says Romanes, "although at first sight this seems 

 self-evident, it is in fact untrue. For, unless the individuals 

 which are indiscriminately isolated happen to be a very large 

 number, sooner or later their progeny will come to differ 

 from that of the parent type, or unisolated portion of the 

 previous stock. And, of course, as soon as this change of 

 type begins, the isolation ceases to be indiscriminate : the 

 previous apogamy [indiscriminate isolation] has been con- 

 verted into homogamy [discriminate isolation], with the 

 usual result of causing a divergence of type. The reason 

 why progeny of an indiscriminately isolated section of an 

 originally uniform stock e. g., of a species will eventually 

 deviate from the original type is, to quote Mr. Gulick, 25 as 

 follows : "No two portions of a species possess exactly the 

 same average character, and, therefore, the initial differ- 

 ences are for ever reacting on the environment and on each 

 other in such a way as to ensure increasing divergence as 

 long as the individuals of the two groups are kept from 

 intergenerating." 



Gulick was led to his recognition of the principle in ques- 

 tion, not by any deductive reasoning from general principles, 

 but by his own particular and detailed observa- 



G-nlick's stud- J 



ies of Hawaiian tions of the land mollusca of the Sandwich 

 land-snails, islands. Here there is an immense number 

 of varieties belonging to several genera ; but every variety 

 is restricted, not merely to the same island, but actually 

 to the same valley. Moreover, on tracing this fauna 

 from valley to valley, it is apparent that a slight varia- 

 tion in the occupants of valley 2 as compared with those 

 of the adjacent valley i, becomes more pronounced in 

 the next valley 3, still more so in 4, etc., etc. Thus it 

 was possible, as Mr. Gulick says, roughly to estimate the 

 amount of divergence between the occupants of any 



