232 Darwin, and after Darwin. 



more and more grown to lay stress on the hereditary 

 character of such peculiarities as they select for 

 diagnostic features of specific distinctness. Indeed 

 it is not too much to say that, at the present time, 

 evolutionists in general recognize this character as, 

 theoretically, indispensable to the constitution of 

 a species. But it is likewise not too much to say 

 that, practically, no one of our systematic naturalists 

 has hitherto concerned himself with this matter. 

 At all events, I do not know of any who has ever 

 taken the trouble to ascertain by experiment, with 

 regard to any of the species which he has consti- 

 tuted, whether the peculiar characters on which his 

 diagnoses have been founded are, or are not, heredi- 

 tary. Doubtless the labour of constituting (or, still 

 more, of reconstituting) species on such a basis of 

 experimental inquiry would be insuperable ; while, 

 even if it could be accomplished, would prove unde- 

 sirable, on account of the * chaos it would produce 

 in our specific nomenclature. But, all the same, we 

 must remember that this nomenclature as we now 

 have it and, therefore, the partitioning of species as 

 we have now made them has no reference to the 

 criterion of heredity. Our system of distinguishing 

 between species and varieties is not based upon the 

 definition which we are now considering, but upon 

 that which we last considered frequently coupled, 

 to some undefinable extent, with No. 2. 



5. There is, however, yet another and closer defini- 

 tion, which may be suggested by the ultra- Darwinian 

 school, who maintain the doctrine of natural selection 

 as the only possible cause of the origin of species, 

 namely : 



