90 BIOLOGY AND SOCIAL PROBLEMS 



of De Vries. According to De Vries the 

 significant hereditary differences between in- 

 dividual organisms, plants as well as animals, 

 depend upon the presence of unit characters, 

 so called, such as those of black or white coat- 

 color in the example of Mendelian inheritance 

 just described. These unit characters were be- 

 lieved by De Vries to be of a stability com- 

 parable with that of the chemical elements ; 

 there were no intergrades. Many of these 

 unit characters are known, and every individ- 

 ual organism can be regarded as a special 

 combination of them. Thus animals and plants 

 can be grouped in accordance with the unit 

 characters that enter into their composition ; 

 all those having the same unit characters con- 

 stitute what De Vries calls an elementary 

 species and when they differ in this respect 

 they belong to different elementary species. 

 Thus, guinea-pigs that differ in coat-color, 

 like the examples previously cited, would con- 

 stitute two elementary species separable by 

 the specific unit characters of black and of 

 white coat-color. It is to be observed that the 

 conception of elementary species is quite dif- 

 ferent from that of the old Linnsean species. 

 Two members of the same elementary species, 



