2 74 THE HISTOEY OF CEEATION. 



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 

 le3n 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 naivete 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, etc. I have proved this in detail in the criti- 

 cism of the idea of species in my " General Morphology " 

 (Gen. Morph. ii. 328-364). I will waste no more time on 

 this unsatisfactory discussion, and now only add a few 

 words about the relation of species to hyhridism. 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 



