18 DISTRIBUTION OF JAYS. [CH. I 



rather than the Ethiopian species, which is not a little 

 remarkable. It has, as have the neotropical forms, the 

 generative openings between the legs of the penultimate 

 pair. 



Separate areas of the species of a genus. 



It sometimes happens that the area of distribution of 

 a genus is perfectly continuous but traversed by large 

 rivers or other checks to distribution. A genus occupying 

 a group of islands, for example, may be said to have an 

 unbroken range so far as is possible ; but here under 

 similar circumstances it is frequently the case that the 

 isolation has been accompanied by the breaking up of the 

 genus into a number of species, perhaps corresponding 

 with the subdivisions of the area. The islands of the 

 group which together constitute the Sandwich Islands are 

 often inhabited by particular species belonging to a genus 

 common to the whole archipelago ; the huge tortoises of 

 the Galapagos are in the same condition. A large tract 

 of country is often similarly inhabited by a series of 

 species belonging to the same genus ; but each of these 

 keeps rigidly to its own particular territory : an inter- 

 mingling is rendered difficult perhaps by the infertility of 

 the species with each other, and partly also by the fact 

 that, the ground being already taken up, there is no room 

 for the inroad of a closely allied form, which has presumably 

 the same or nearly the same mode of life and would 

 therefore seriously compete. The twelve species of Jays 

 belonging to the genus Garrulus range over the greater 

 part of the Palsearctic region ; but nearly every species 



