274 THE HISTORY OF CREATION, 
whether this or that observed form is a species or a variety, 
whether it is a really good or a bad species. The most 
general answer to this question used to be the following: 
“To one species belong all those individuals which agree in 
all essential characteristics. Essential characteristics of 
species are those which remain permanent or constant, and 
never become modified or vary.” But as soon as a case 
occurred in which the characteristic—which had hitherto 
been considered essential—did become modified, then it was 
said, “This characteristic is not essential to the species, for 
essential characteristics never vary.” Those who argued 
thus evidently moved in a circle, and the naiveté with 
which this circular method of defining species is laid down 
in thousands of books as an unassailable truth, and is still 
constantly repeated, is truly astonishing. 
All other attempts which have been made to arrive at a 
definite and logical determination of the idea of organic 
“species ” have, like the last, been utterly futile, and led to 
no results. Considering the nature of the case, it cannot be 
otherwise. The idea of species is just as truly a relative 
one and not absolute, as is the idea of variety, genus family, 
order, class, ete. I have proved this in detail in the criti- 
cism of the idea of species in my “General Morphology ” 
(Gen. Morph. ii. 323-364). I will waste no more time on 
this unsatisfactory discussion, and now only add a few 
words about the relation of species to hybridism. Formerly 
it was regarded as a dogma, that two good species could 
never produce hybrids which could reproduce themselves as 
such. Those who thus dogmatized almost always appealed 
to the hybrids of a horse and donkey, the mule and the 
hinny, which, truly enough, are seldom able to reproduce 
