XI PHENOMENA OF ORGANIC NATURE 419 



call distinct SPECIES of pigeons ; that is to say, 

 that they differ so much in structure that there is 

 a greater difference between the Pouter and the 

 Tumbler than there is between such wild and dis- 

 tinct forms as the Rock Pigeon or the Ring Pigeon, 

 or the Ring Pigeon and the Stock Dove ; and 

 indeed the differences are of greater value than 

 this, for the structural differences between these 

 domesticated pigeons are such as would be ad- 

 mitted by a naturalist, supposing he knew nothing 

 at all about their origin, to entitle them to con- 

 stitute even distinct genera. 



As I have used this term SPECIES, and shall prob- 

 ably use it a good deal, I had betterperhaps devote 

 a word or two to explaining what I mean by it. 



Animals and plants are divided into groups, 

 which become gradually smaller, beginning with 

 a KINGDOM, which is divided into SUB-KINGDOMS ; 

 then come the smaller divisions called PROVINCES ; 

 and so on from a PROVINCE to a CLASS, from a 

 CLASS to an ORDER, from ORDERS to FAMILIES, 

 and from these to GENERA, until we come at 

 length to the smallest groups of animals which 

 can be defined one from the other by constant 

 characters, which are not sexual; and these are 

 what naturalists call SPECIES in practice, whatever 

 they may do in theory. 



If, in a state of nature, you find any two groups 

 of living beings, which are separated one from the 

 other by some constantly-recurring characteristic, 



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