1G4 THE THEORIES OF EVOLUTION 



Organicism is not so much a theory of heredity as 

 a theory of ontogenesis. Roux and the other organi- 

 cists lay special stress on the factors of individual evo- 

 lution. To them the main question is the origin of 

 ontogenetic, anatomic and histological differentiation. 

 Weismann and his disciples considered the cells as po- 

 tentially differentiated, at the time of the cleavage of 

 the mother cell, by purely internal factors. The biol- 

 ogists of the organic school, among them O. Hertwig, 

 Herbst, Loeb, Driesch (the last named in his earlier 

 works only, as his vitalist studies led him later to draw 

 different conclusions) , locate the factors of differenti- 

 ation outside of the cell. 



Hertwig, for instance, considers the successive 

 cleavage of the ovum as invariably homogeneous and 

 makes the differentiation of a cell dependent upon its 

 location relatively to the surrounding cells. "Differ- 

 entiation," he says, "is due to location. In the gastrula 

 it is not the endoderm which becomes invaginated; it 

 is whatever becomes invaginated which constitutes the 

 endoderm." In other words the mere fact that 

 certain cells constitute the internal wall of the 

 gastrula causes these cells to assume endodermic 

 characters. 



Others, Hartog, Roux and Kopsch, attribute differ- 

 entiation to the influence of the various tropisms and 

 tactisms, such as the mutual attraction of blastomeres 

 and of cells, an attraction which Roux designates as 



