PERPETUATION OF LIVING BEINGS. 109 



between these domesticated pigeons are such as would 

 be admitted by a naturalist^ supposing he knew nothing 

 at all about their origin, to entitle them to constitute 

 even distinct genera. 



As I have used this term Species, and shall probably 

 use it a good deal, I had better perhaps 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, I don't 

 care how slight and trivial, so long as it is defined 

 and constant, and does not depend on sexual pecu- 

 liarities, then all naturalists agree in calling them two 

 species ; that is what is meant by the use of the word 

 species — that is to say, it is, for the practical naturalist, 

 a mere question of structural differences.^ 



We have seen now — to repeat this point once more, 

 and it is very essential that we should rightly under- 



* I lay stress here on the practical signification of "Species." 

 Whether a physiological test between species exist or not, it is 

 hardly ever applicable by the practical naturalist. 



