244 Fertilization and Hyhridimtion 



be obtained only when the interchange is limited to like 

 units. 



We distinguish here specific characteristics from indi- 

 vidual features. The units in the hereditary substance 

 of the nuclear thread compose the former. Every species 

 has an often exceedingly large and yet definite and invari- 

 able number of them. The sum total of these units 

 forms that which distinguishes any given species from all 

 others, even from its nearest allies. A complete diagno- 

 sis of a species would have to embrace all of these char- 

 acteristics, and therewith all the material bearers under- 

 lying them. 



The individual features, that is, the differences be- 

 tween the individuals within the species, and not only of 

 the systematic but of the so-called elementary species, are 

 of quite another nature. It is true that they are, in a way, 

 hereditary, but with that they are subject to constant 

 changes. The average stature of man remains the same 

 in the course of centuries, for the same race (elementary 

 species), but the individual stature changes constantly 

 from one individual to another. In the somatic cells of 

 man the bearers of the stature of the father lie opposite 

 those of the mother. At the moment of exchange these 

 are mutually transferred, and the sexual cells receive 

 partly one, partly the other stature, but this in the most 

 various combinations with the other characters. Thus 

 one might continue. Every visible quality, every trait 

 of character is to be found in all individuals, only in some 

 they are strongly developed and prominent, in others 

 weak and recessive. Ordinary observation takes more 

 interest in differences than in similarities, and for this 

 reason the former are designated by contrasting expres- 

 sions, as large and small, strong and weak, forward and 



