II THE ORIGIN OF SPECIES 45 



but the offspring of such crossed races are 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 individuals 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 off- 

 spring is fertile with others produced in the same 

 way, the groups are races and not species. If, on 

 the 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 suscep- 

 tible of a definite interpretation. Unfortunately, 



