168 THE THEORIES OF EVOLUTION 



in the organism) is the main cause of ontogenetic 

 differentiation, chemical and functional. 



A similar struggle is waged between cells, for cells 

 also react differently when stimulated, and the space 

 they can occupy in Hie organism is limited. The cells 

 that are best fitted to multiply overpower the other 

 cells. Differentiation then becomes more marked, for 

 whenever cells of the same order are characterised by 

 the presence of the same substance, those in which this 

 substance exists in larger quantities multiply more 

 rapidly. 



The same struggle goes on between tissues and even 

 between organs, with this difference, however, that 

 organic necessity sets definite limits, and that the 

 exaggerated development of certain organs could be 

 harmful to the organism and cause it to be eliminated 

 by natural selection. The struggle is not carried be- 

 yond the limits within which it contributes to the 

 economical utilisation of food and space. 



At the time when the cells become definitely differ- 

 entiated there intervenes another factor of ontogene- 

 sis: functional stimulation. It is absolutely depend- 

 ent, however, upon cellular differentiation; for after 

 a certain stimulation has favoured in a cell the assim- 

 ilation of a certain substance at the expense of others, 

 and all other substances have been gradually elimi- 

 nated, the cell is only sensitive to this particular kind 

 of stimulation. The real function of the cell consists 



