

PERPETUATION OF LIVING BEINGS 81 



and beaks, and so on ; but that there might be an absolute 

 difference in the number of the vertebrae of the back, as 

 in the sacral vertebrae of the Pouter ; and so great is the 

 extent of the variation in these and similar characters 

 that I pointed out to you, by reference to the skeletons 

 and the diagrams, that these extreme varieties may 

 absolutely differ more from one another in their structural 

 characters than do what naturalists 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 distinct 

 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 

 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 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 



* 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. 



