44 THE ORIGIN OF SPECIES n 



to be obvious ; for if, as the result of spontaneous 

 variation and of selective breeding, the progeny of 

 a common stock may become separated into groups 

 distinguished from one another by constant, not 

 sexual, morphological characters, it is clear that 

 the physiological definition of species is likely to 

 clash with the morphological definition. No one 

 would hesitate to describe the pouter and the 

 tumbler as distinct species, if they were found fossil, 

 or if their skins and skeletons were imported, as 

 those of exotic wild birds commonly are and with- 

 out doubt, if considered alone, they are good and 

 distinct morphological species. On the other hand, 

 they are not physiological species, for they are 

 descended from a common stock, the rock-pigeon. 



Under these circumstances, as it is admitted on 

 all sides that races occur in Nature, how are we to 

 know whether any apparently distinct animals are 

 really of different physiological species, or not, 

 seeing that the amount of morphological difference 

 is no safe guide ? Is there any test of a physio- 

 logical species ? The usual answer of physiologists 

 is in the affirmative. It is said that such a test is 

 to be found in the phenomena of hybridisation 

 in the results of crossing races, as compared with 

 the results of crossing species. 



So far as the evidence goes at present, in- 

 dividuals, of what are certainly known to be mere 

 races produced by selection, however distinct they 

 may appear to be, not only breed freely together, 



