OBJECTIONS AGAINST EVOLUTION. 187 



they should be reckoned not only as distinct species, 

 but also as distinct genera, but because they are fer- 

 tile when crossed inter se, they must be regarded, anti- 

 evolutionists insist, as all belonging to one and the 

 same species. And for this reason, too, we are told 

 that the species of any given organism is to be de- 

 termined, not by its form, but by its filiation. Ac- 

 cording to this view, therefore, the determining 

 characteristic of species is not something morpholog- 

 ical, as Tournefort opined, but rather something, as 

 Ray and Flourens taught, which is physiological. 



But even physiological species is not the con- 

 stant quantity it is represented to be by anti-trans- 

 formists. Infertility of species and of their hybrid 

 progeny does not constitute the positive line of 

 demarcation, so often claimed by the advocates of 

 the immutability of specific forms. On the con- 

 trary, as Darwin and others have shown, " neither 

 sterility nor fertility affords any certain distinction 

 between species and varieties." Long-continued 

 experiments, of the most ingenious character, have 

 demonstrated beyond question that sterility in ani- 

 mals is not to be regarded as an indelible charac- 

 teristic, but as one capable of being removed by 

 domestication. And, observations on numberless 

 groups of plants and animals have disclosed the 

 remarkable fact, that " the degree of fertility, both 

 of first crosses and of hybrids, graduates from zero 

 to perfect fertility." 



From the foregoing, then, it is evinced that physi- 

 ological species present as many and as grave diffi- 

 culties as do morphological species. If it be true, 



