272 LAY SERMONS, ADDRESSES, AND REVIEWS. [xn. 



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, without doubt, if 

 considered alone, they are good and distinct morpho- 

 logical species. On the other hand, they are not physio- 

 logical 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 physiologists 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 com- 

 pared 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 greyhouud, 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 



