268 STUDIES, SCIENTIFIC AND SOCIAL chap. 



the main general conclusions arrived at by naturalists, our 

 purpose will be best attained by proceeding at once to 

 consider his special cases of difficulty ; and in doing so we 

 shall have occasion to discuss, as fully as may be required, 

 the general principles and particular illustrations needed 

 to elucidate them. 



We have first the case of the Little Blue Magpie of 

 Spain (Cyano'pica Cooki), which has a very close ally in the 

 extreme parts of Eastern Asia and Japan {G. cyancus), 

 while there is nothing closely allied to these in all the 

 intervening regions or in any other part of the world. 

 This is said to be an infringement of the canon as to 

 the continuity of specific areas, and as such to require 

 explanation. Before proceeding further, it will be well to 

 inquire into the value of this canon of continuity, and 

 whether it is so clear and well established that infringe- 

 ments of it are altogether exceptions to the usual course 

 of nature. So far from this being the case, I believe it 

 will be found that, between the complete continuity of the 

 area occupied by a species or a genus and such wide dis- 

 continuity as occurs in the present example, there is every 

 possible gradation ; and further, that the instances of dis- 

 continuity are very numerous, while those of complete 

 continuity are far less generally the rule than appears at 

 first sight. 



^In order to understand the bearing of this class of 

 phenomena on the theory of derivation by modification 

 from an allied form, let me briefly indicate the probable 

 course of a genus of animals from its birth or origin to its 

 final extinction. 



Genera are groups of species which agree among them- 

 selves, and differ from all other groups in the same family 

 or order, by the possession of some structural peculiarities. 

 We must therefore suppose a genus to have had its origin 

 in some variation of strttctiire which was useful to its 

 possessors — such as a modification of the bill, feet, or 

 wings of a bird, or of the teeth, claws, or horns of a 

 mammal. According to the theory of natural selection, 

 the possessors of such a useful pecidiarity would increase 

 at the expense of their close allies who did not possess it, 



