156 THE ORIGIN OF SPECIES 



species, if they were found fossil, or if their skins and 

 skeletons were imported, as those of exotic wild birds 

 commonly are and without 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 physiological species ? The usual answer of physio- 

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

 is to be found in the phenomena of hybridization in the 

 results of crossing races, as compared with the results of 

 crossing species. 



So far as the evidence goes at present, individuals, of 

 what are certainly known to be mere races produced by 

 selection, however distinct they may appear to be, not 

 only breed freely together, but the offspring of such crossed 

 races are only perfectly fertile with one another. Thus, 

 the spaniel and the greyhound, the dray-horse and the 

 Arab, the pouter and the tumbler, breed together with 

 perfect freedom, and their mongrels, if matched with other 

 mongrels of the same kind, are equally fertile. 



On the other hand, there can be no doubt that the in- 

 dividuals of many natural species are either absolutely 

 infertile, if crossed with individuals of other species, or, 

 if they give rise to hybrid offspring, the hybrids so produced 

 are infertile when paired together. The horse and the ass, 

 for instance, if so crossed, give rise to the mule, and there 

 is no certain evidence of offspring ever having been produced 

 by a male and female mule. The unions of the rock-pigeon 

 and the ring-pigeon appear to be equally barren of result. 

 Here, then, says the physiologist, we have a means of 

 distinguishing any two true species from any two varieties. 

 If a male and a female, selected from each group, produce 

 offspring, and that offspring is fertile with others produced 

 in the same way, the groups arc races and not species. If, 

 on I he other hand, no result ensues, or if the offspring are 

 infertile with others produced in the same way, they are 

 true physiological species. The test would be an admirable 

 one, if, in the first place, it were always practicable to 

 apply it, and if, in the second, it always yielded results 



