252 DARWINISM TO-DAY. 



two given valleys by measuring the number of miles be- 

 tween them. 



On the basis of his detailed observations, Gulick 26 has 

 proposed the following three general propositions as to the 

 relations of isolation to species-forming: 



"i. A species exposed to different conditions in the differ- 

 ent parts of the area over which it is distributed, is not 

 represented by divergent forms when free inter-breeding 

 exists between the inhabitants of the different districts. In 

 other words, diversity of natural selection without separation 

 does not produce divergent evolution. 



"2. We find many cases in which areas, corresponding in 

 the character of the environment, but separated from each 

 other by important barriers, are the homes of divergent 

 forms of the same or allied species. 



"3. In cases where the separation has been long continued, 

 and the external conditions are the most diverse in points 

 that involve diversity of adaptation, there we find the most 

 decided divergences in the organic forms. That is, where 

 separation and divergent selection have long acted, the re- 

 sults are found to be the greatest. 



"The first and second of these propositions will probably 

 be disputed by few 7 , if by any. The proof of the second is 

 found wherever a set of closely allied organisms is so dis- 

 tributed over territory that each species and variety occu- 

 pies its own narrow district, within which it is shut by bar- 

 riers that restrain its distribution, while each species of the 

 environing types is distributed over the whole territory. 

 The distribution of terrestrial molluscs on the Sandwich 

 Islands presents a great body of facts of this kind." 



Finally in a recent exhaustive discussion of the subject 27 

 of the relation of isolation to evolution Gulick declares that 

 "the whole process of bionomic evolution, whether progres- 

 sive or retrogressive, whether increasingly ramified and 1 

 divergent, or increasingly convergent through amalgama- 



