376 THE EVOLUTION THEOKY 



It is obvious that, if the principle of selection operates in nature 

 at all, it must do so wherever living units struggle together for the 

 same requirements of life, for space and food, and these units n©ed 

 not be persons, but may represent every category of vital units, from 

 the smallest invisible units up to the largest. For in all these cases 

 the conditions of the selection-process are given: individual variability, 

 nutrition, and multiplication, transmission of the advantage attained, 

 and, on the other hand, limitation of the conditions of existence — 

 especially food and space. The resulting struggle for existence must, 

 in every category of vital units, be most acute between the individual 

 members of each category, as Darwin emphasized in the case of 

 species from the yery first, and persistent variations of a species of 

 living units can only be brought about by this kind of struggle. 

 Strictly speaking, therefore, we should distinguish as many kinds of 

 selection-processes as there are categories of living units, and these 

 could not be sharpl}^ separated from one another, apart from, the fact 

 that we have to infer many of them, and cannot recognize their 

 gradations. Here, as everywhere else, we must break up the 

 continuity of nature into artificial groups, and . it seems best to 

 assume and distinguish between four main grades of selective 

 processes corresponding to the most outstanding and significant 

 categories of vital units, namely : Germinal, Histonal, Personal, and 

 Cormal Selection. 



Histonal Selection includes all the processes of selection which 

 take place between the elements of the body (soma), as distinguished 

 from the germ-plasm, of the Metazoa and Metaphyta, not only between 

 the ' tissues ' in the stricter sense, but also between the parts of the 

 tissues, that is, the lower vital units of which they are composed, 

 and which Wilhelm Roux, when he published his Kamj^f der Telle 

 (' Struggle of the Parts '), called ' molecules.' It occurs between all the 

 parts of the tissues down to the lowest vital units, the biophors. 

 We must also reckon under histonal selection the processes of selection 

 which take place between the elements of the simplest organisms, 

 and through which these have gradually attained to greater 

 complexity of structure and increased functional caj^acity. As long- 

 as no special hereditary substance had been differentiated, variations 

 which arose in the simplest organisms through selection-processes of 

 this kind were necessarily transmitted to the descendants, but after 

 this differentiation had taken place this could no longer occur — 

 ' acquired ' modifications of the soma were no longer transmitted, and 

 the importance of histonal selection was limited to the individual. 

 But this form of selection must be of the greatest importance in 



