I THE DARWINIAN HYPOTHESIS 



described and may be identified in collections, and 

 the number of separable kinds of living things is 

 under-estimated at half a million. Seeing that 

 most of these obvious kinds have their accidental 

 varieties, and that they often shade into others 

 by imperceptible degrees, it may well be 

 imagined that the task of distinguishing be- 

 tween what is permanent and what fleeting, 

 what is a species and what a mere variety, 

 is sufficiently formidable. 



But is it not possible to apply a test whereby a 

 true species may be known from a mere variety ? 

 Is there no criterion of species ? Great authori- 

 ties affirm that there is that the unions of 

 members of the same species are always fertile, 

 while those of distinct species are either sterile, 

 or their offspring, called hybrids, are so. It is 

 affirmed not only that this is an experimental 

 fact, but that it is a provision for the preservation 

 of the purity of species. Such a criterion as this 

 would be invaluable ; but, unfortunately, not only 

 is it not obvious how to apply it in the great 

 majority of cases in which its aid is needed, but 

 its general validity is stoutly denied. The Hon. 

 and Rev. Mr. Herbert, a most trustworthy authority, 

 not only asserts as the result of his own observa- 

 tions and experiments that many hybrids are 

 quite as fertile as the parent species, but he goes 

 so far as to assert that the particular plant Crinum 

 capense is much more fertile when crossed b 



