296 THE EVOLUTION THEORY 



and in proportion to the number of unoccupied areas which are open 

 to the descendants of the immigrant species. This is undoubtedly the 

 reason why the Sandwich Islands do not possess a single s^^ecies which 

 occurs elsewhere, and the segregation of the unknown ancestral form 

 into many species and several (four) sub-genera is also to be inter- 

 preted in the same way. There was probably in this case only one 

 immigrant species, which found a free field, and adapted itself in its 

 descendants to all the conditions of snail-life which obtained there, 

 and in doing so split up into numerous and somewhat markedly 

 divergent forms. But the number of different forms is much greater 

 than the number of distinctive habitats, as Gulick indicates and sub- 

 stantiates in detail, for similar areas, if they are relatively isolated from 

 one another, are inhabited not by the same forms, but by different 

 though nearly related varieties, and this depends on the fact that from 

 the species which was in process of varying a different combination of 

 variations would be sent out at different periods, and the temporary 

 isolation would result in the evolution of special local varieties. 



But I do not believe that this would continue for all time. I 

 rather think that these — let us say — representative varieties would 

 diminish in numbers in the course of a long period. For the isolation 

 of single valley-slopes or of particular woods is not permanent, indi- 

 viduals are liable to be carried from one to another in the course of 

 centuries as they were at the beginning of the colonization of the 

 isolated woods ; forests are cleared or displaced by geological changes, 

 connexions are formed between places which were formei^ly separated, 

 and in the course of another geological period the number of repre- 

 sentative varieties, and probably even of species, will have diminished 

 considerably, — the former will have been fused together, the latter 

 in part eliminated. Even now Gulick speaks regretfully of the 

 decimation of rare local forms by their chief enemies, the mice. 



But even if the number of endemic forms in insular regions 

 diminishes from the time when they were first fully taken possession 

 of, ij nevertheless remains a very high one, for even now Madeira 

 possesses 104 endemic terrestrial snails, the Philippines have more 

 than the whole of India, and the Antilles as man}^ as the whole 

 American continent. 



Many naturalists believe that each isolated variety must diverge 

 further and further from its nearest relatives as time goes on. 

 Although I entirely admit that this is possible, for I have 

 endeavoured to show that variational tendencies which have once 

 arisen in the germ-plasm go on in the same direction until they are 

 brought to a full stop in some way or otlier, yet I cannot admit that 



