xii.] THE ORIGIN OF SPECIES. 237 



are certainly known to be mere races produced by selection &amp;gt; 

 however distinct they may appear to be, not only breed freely 

 together, but the offspring of such crossed races are only per 

 fectly 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 offspring is fertile with others pro 

 duced 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 susceptible of a definite 

 interpretation. Unfortunately, in the great majority of cases, 

 this touchstone for species is wholly inapplicable. 



The constitution of many wild animals is so altered by con 

 finement that they will not breed even with their own females, 

 so that the negative results obtained from crosses are of no value ; 

 and the antipathy of wild animals of different species for one 

 another, or even of wild and tame members of the same species, 

 is ordinarily so great, that it is hopeless to look for such unions 

 in Nature. The hermaphrodism of most plants, the difficulty 

 in the way of insuring the absence of their own, or the proper 



