THE MECHANISM OF LIFE 573 



fact. That this dependence, the higher we ascend in the develop- 

 mental series of organisms, becomes closer and more fixed, depends 

 again solely upon utility, for the greater the unity in the govern- 

 ment of the whole community, the surer and greater is the work 

 of the whole, and the greater also is the advantage that the 

 individual cell receives from the common life. Unity in the 

 government of the cell-community is, however, determined essen- 

 tially by the dependent relation of the individual cell to the 

 other cells. Darwin's theory of selection, which contains a general 

 explanation of adaptation in the organic world, has made it clear 

 how such adaptative arrangements must be developed in a natural 

 manner. Of course, the immediate mechanical causes are to be 

 sought in each individual case. 



B. DIFFERENTIATION AND DIVISION OF LABOUR AMONG THE CELLS 



In the evolution of mutually dependent relations between the 

 cells in the origin of the cell-community we have become acquainted 

 with only one result of the common life of the cells. It is the sole 

 result, so long as the community does not surpass certain dimen- 

 sions. If, however, the community becomes larger, if it develops 

 into a compact mass, another necessary mechanical result of the 

 association is observed, namely, the differentiation of and division 

 of labour among the cells. 



The differentiation of cells consists, as is well known, in the as- 

 sumption by the cells of different characters, so that the community 

 is no longer composed of like cells, but of cells and cell-groups of 

 different kinds. Therewith there appear not only morphological, 

 but also physiological differences between the individual cells, i.e., 

 the performances of certain cells or cell-groups become different 

 from those of others, and a division of labour between them takes 

 place. Differentiation and division of labour are inseparable from 

 one another. 



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

 munity are fairly evident. All the properties of an organism, 

 morphological as well as physiological, are the expression of the 

 interaction of two factors, namely, the relations between its internal 

 and its external vital conditions. 1 If one of these two factors 

 changes, there is a change of the properties of the organism. If, 

 therefore, a cell divides into many like offspring, and if all these 

 offspring remain together and form a cell-community, all the con- 

 stituents of this community will remain alike, so long as the 

 external conditions surrounding each cell are the same as those 

 surrounding all the others. We have become acquainted with 

 such cell-communities among the Protista. But such a community 

 is only possible when all the cells are arranged beside one another 



1 Cf. p. 297. 



