138 WHAT EVOLUTION IS 



black guinea pig differs from a white 

 guinea pig in one unit character, and 

 yet this is sufficient to place these two 

 individuals in different elementary 

 species. This is certainly a novel 

 conception, for it implies that two 

 brothers may be of diverse species 

 provided they show a unit character 

 difference. In evolution, however, 

 we are not so much concerned with 

 this aspect of the subject as with 

 another. 



As already stated, the unit char- 

 acters, by which elementary species 

 may be distinguished, show no inter- 

 grades; they are fixed characters. 

 Hence the difference between one ele- 

 mentary species and another is an 

 abrupt difference. These abrupt dif- 

 ferences are what de Vries calls muta- 

 tions, and he contrasts them with 

 the very slight individual differences 

 which are seen between members of 



