30 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 are 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—t.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. 
