244 Darwin, and after Darwin. 



correlation between the birth of new species and the 

 immediate pre-existence of closely allied species on 

 the same area or, at most, on closely contiguous areas. 



Where a continuous area has long been circum- 

 scribed by barriers of any kind, which prevent the 

 animals from wandering beyond it, then we find that 

 all the species, both extinct and living, constitute 

 more or less a world of their own ; while, on the 

 other hand, where the animals are free to migrate 

 from one area to another, the course of their migra- 

 tions is marked by the origination of new species 

 springing up en route, and serving to connect the 

 older, or metropolitan, forms with the younger, or 

 colonising, forms in the way of a graduated series. 

 This principle, however, admits of being traced only 

 in certain cases of species belonging to the same 

 genus, of genera belonging to the same family, or, 

 at most, of families belonging to the same order. 

 In other words, the more general the structural 

 affinity, the more general is the geographical ex- 

 tension as we should expect to be the case on the 

 theory of descent with branching modifications, seeing 

 that the larger, the older, and the more diverse the 

 group of organisms compared, the greater must be 

 their chances of dispersal. 



These general considerations led us to contemplate 

 more in detail the correlation between structural 

 affinity and barriers to free migration. Such barriers, 

 of course, differ in the cases of different organisms. 

 Marine organisms are stopped by land, unsuitable 

 temperature, or unsuitable depths ; fresh-water or- 

 ganisms by sea and by mountain-chains ; terrestrial 

 organisms chiefly by water. Now it is a matter of 



