XIT.] Cfr* rigiir 0f Species. 273 



be equally barren of result. Here, then, says the phy- 

 siologist, we have a means f 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 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 admir- 

 able 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. Unfor- 

 tunately, in the great majority of eases, this touchstone 

 for species is wholly inapplicable. 



The constitution of many wild animals is so altered 

 by confinement 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 ensuring the absence of their own, or the proper 

 working of other pollen, are obstacles of no less magni- 

 tude in applying the test to them. And in both animals 

 and plants is superadded the further difficulty, that 

 experiments must be continued over a long time for the 

 purpose of ascertaining the fertility of the mongrel or 

 hybrid progeny, as well as of the first crosses from 

 which they spring. 



Not only do these great practical difficulties lie in the 

 way of applying the hybridization test, but even when 

 this oracle can be questioned, its replies are sometimes 

 as doubtful as those of Delphi. For example, cases are 

 cited by Mr. Darwin, of plants which are more fertile, 



T 



