76 ISOLATION AS A FACTOR 



Leicester sheep, those of Mr. Buckley and of Mr. 

 Burgess, were " purely bred from the original 

 stock of Mr. Bakewell for fifty years." There is 

 not a suspicion of a single instance of deviation 

 from the pure Bakewell Leicester breed in either 

 flock. Yet after fifty years the difference in the 

 flocks of sheep is so great that they " have the 

 appearance of being quite different varieties." 

 In nature, as in domestication, individuals of the 

 same race, animals or plants, prevented from 

 inter-crossing for a long time, present at least the 

 appearance of distinct varieties or species. A 

 study of the weeds of the world, as they have 

 spread from place to place, should show this fact 

 in interesting fashion. It can also be shown by 

 a comparative study of dogs or horses. Mr. 

 Vernon Bailey tells me that in the pouched 

 gophers and other rodent groups each valley has 

 its individual peculiarities, those shown in the 

 skulls as well as in the forms or colors of the ani- 

 mals. All these variations, too small to justify 

 the use of technical names, form the beginnings 

 of difference in subspecies. With more perfect 

 isolation these characters would soon assume 

 greater importance. They seem to indicate the 

 beginning of species-forming. 



So far as species in nature are concerned, we 

 can account for the origin of none of them, except 

 on the ground of the presence of some forms of 

 isolation. In those groups of animals or plants 

 which have been most studied, subspecies or vari- 



