THE ORIGIN OF SPECIES 157 



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 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 insuring the absence of their own, or the proper working 

 of other pollen, are obstacles of no less magnitude 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 with 

 the pollen of another species than with their own ; and 

 there are others, such as certain fuci, whose male element 

 will fertilize the ovule of a plant of distinct species, while 

 the males of the latter species are ineffective with the 

 females of the first. So that, in the last-named instance, a 

 physiologist, who should cross the two species in one way, 

 would decide that they were true species ; while another, 

 who should cross them in the reverse way, would, with 

 equal justice, according to the rule, pronounce them to be 

 mere races. Several plants, which there is great reason 

 to believe are mere varieties, are almost sterile when 

 crossed ; while both animals and plants, which have 

 always been regarded by naturalists as of distinct species, 

 turn out, when the test is applied, to be perfectly fertile. 

 Again, the sterility or fertility of crosses seems to bear no 

 relation to the structural resemblances or differences of 

 the members of any two groups. 



Mr. Darwin has discussed this question with singular 

 ability and circumspection, and his conclusions are summed 

 up as follows, at page 276 of his work : 



