50 THE THEORY OF NATURAL SELECTION 



order to complete the Darwinian account of the origin 

 of species, is that of segregation or isolation. If the 

 selection of minute fortuitous variations in different 

 directions is capable of breaking up a species into a 

 number of new species, it seems clear that this can 

 only happen when the members of the different branches 

 are prevented from interbreeding ; since otherwise the 

 effect of selection would be counteracted by the 

 mingling or blending of characters which may be sup- 

 posed to result from free intercrossing. Further, many 

 zoologists, and more especially the systematists among 

 them, believe that isolation in itself has a most im- 

 portant function in modifying species. This isolation 

 may be either geographical, as when distance or some 

 physical barrier separates different members of the 

 same species ; or it may be physiological, as when 

 structural or temperamental differences, or mutual 

 distaste, prevent the mating of certain individuals. 



The researches of Gulick upon the species of snails 

 found in the Haiwaian Islands showed that the differ- 

 ences between the species correspond in amount with 

 their degree of separation in space i.e., with their 

 isolation. The characters which separate these species 

 could not be shown to have any relation to differences 

 in the environment, since adjoining valleys, which 

 differed considerably in vegetation and rainfall, pos- 

 sessed closely related species ; whilst in valleys further 

 apart, but more similar in the environment offered 

 to the snails, the characteristic species showed much 

 greater differences. The case of the Haiwaian snails, 

 therefore, appears to afford an exception to the prin 

 ciple next to be described. 



