298 THE EVOLUTION THEORY 



germinal selection, natural selection, and sexual selection. We can 

 say generally that each grade and mode of selection will more readily 

 lead to the transformation if it be combined with isolation. Thus 

 germinal selection may call forth slight divergences in colour and 

 marking, which will be permanent if the individuals concerned are in 

 an isolated region. In isolation these variations will increase undis- 

 turbed, and in some circumstances will be intensified b}^ sexual 

 selection, so that the male sex will vary alone in the first place, 

 though the female ma\^ follow^ so that ultimately the whole species 

 will be transformed. Finally, the most marked effect of isolation is 

 seen when individual members of a species are transferred to virgin 

 territory which oflfers unoccupied areas, suitable not to one particular 

 species alone, but to many nearly related species, so that the immi- 

 grant colony can adapt itself to all the diflferent possibilities of life, 

 and develop into a whole circle of species. But we saw that such an 

 aftergrowth of new forms, whetlier varieties, species, or even genera, 

 may far exceed the number of different kinds of localities, if there be 

 relative isolation between the diflferent groups of immigrants within 

 the insular region, as happens in the case of slow-moving animals like 

 the terrestrial snails, or of small singing-birds, to which each island of 

 a little archipelago is a relatively isolated region (Galapagos). 



We may thus fully recognize the importance of local isolation 

 without regarding the absence of crossing with the members of the 

 species in the original habitat as the sole cause of species-formation, 

 without setting ' isolation ' in the place of the processes of selection. 

 These last, taken in the wide sense, always remain the indispensable 

 basis of all transformations, but they certainly do not operate 

 only in the form of personal selection, but, wherever indifferent 

 characters are concerned, in that of germinal selection. Here, too, we 

 see the possibility of reconciliation with those naturalists who regard 

 transformations as primarily dependent upon internal forces of 

 development. The fact is that all variations depend upon internal 

 causes, and their course must be guided by forces which work in an 

 orderly w^ay. But the actual co-operation of all these forces and 

 variations is not predetermined, but depends to a certain extent upon 

 chance, for of the possible modes of evolution the one which gains the 

 upper hand in the play of forces at the moment is alone followed, the 

 better are everywhere preferred, from the most minute vital units of 

 the germ-plasm, up to the struggle between individuals and between 

 species. 



