104 CONDITIONS AFFECTING THE 



As I have used this term Species, and shall proba- 

 bly 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 peculiarities, 

 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- 

 stand it — we have seen that breeds, known to have 

 been derived from a common stock by selection, may 

 be as different in their structure from the original stock 

 as species may be distinct from each other. 



But is the like true of the physiological charac- 



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

 er a physiological test between species exist or not, it is hardly ever ap- 

 plicable by the practical naturalist. 



