THE MECHANISM OF LIFE 575 



the animal and plant cell-community. This principle is here 



realised most completely and in the smallest details, and finally 



leads to the construction of so complex an organism as the cell- 



community of the human body. The whole development of the 



most complex animal body with all its differentiations depends 



solely upon the principle that the farther cell-increase proceeds 



upon the simple mechanical basis of the different relative positions 



of the cells and cell-groups arising from the continued division of 



the ovum, the more various must be the mutual relations and the 



external vital conditions of these cells and cell-groups, so that, by 



adaptation to the constantly changing external conditions, the cells 



and cell-groups finally diverge and become gradually differentiated 



as regards all their characteristics. As is known from the funda- 



mental law of biogene- 



sis, the mechanics of on- 



togenetic development, 



in so far as special ad- 



aptations do not come 



into play, pursues es- 



sentially the same 



course that develop- 



ment has pursued in 



the phylogenetic series. 1^ 



The mechanical causes m> 



of the differentiation of ^C^~ ji 



the cells in the forma- 



* 



munity are evidently 

 the same in their most 



essential features in FlG - Z'n.Prototpongia H<KckelU. (After Lang.) 



the ontogeny as in the 



phylogeny. It remains for the embryology of the future to discover 

 in detail the very manifold special relations, which are as different 

 as the organisms themselves. 



While the mechanical causes of cell-differentiation in the com- 

 plicated cell-community must be sought in changes of its relations 

 with the environment, which for every cell and cell -generation are 

 due to continued cell-division, division of labour among the cells is 

 based upon the development of the cell-community itself 1 The work 

 of every multicellular organism is the expression of the activity of 

 its individual cells. If the cells are different, they contribute in a 

 different manner to the whole labour of the organism. That this 

 combined labour must become harmonious and advantageous 

 follows from the principle of selection, which controls all organic 

 development, phylogenetic as well as ontogenetic. Only those 

 cell-communities continue to live, in which the cell-generations 



1 Of. p. 536. 



> 

 tion of every cell-corn- / Y \^ V f 



\ \ 



