CAUSE3 OF EVOLUTION 383 



organization shows that the first condition of higher de- 

 velopment and civilization is on the one hand the division 

 of the various duties among the various classes of the 

 citizens and on the other hand, the co-operation of these 

 individuals for the common purpose of the state. This 

 is exactly the case also in every many celled organism. 

 Every multi-cellular individual in the plant or animal 

 kingdom is more perfectly developed and ranks higher in 

 proportion as the division of labor among its constituent 

 cells, the differentiation of its cell individuals is more 

 perfect. Therefore in the various classes of organisms, 

 we find this differentiation, sometimes in a more, some- 

 times in a less perfect condition. The simplest form of 

 division of labor occurs in those lower animals in the 

 bodies of which only two kinds of cells have become dif- 

 ferentiated. This is the case for example, in the lowest 

 plant animal, in sponges and the simplest polyps, as well 

 as in their common parent form, the gastraea. 



"Throughout the entire many celled bodies of these 

 there are only two different kinds of cells, the one kind 

 affect nutrition and reproduction of the animal, the other 

 kind are its organs of feeling and motion. These two 

 kinds of cells are identical with those which first come to 

 perfection in the first process of differentiation of the 

 germ layers in the human embyro, but in most higher 

 animals, the differentiation of the cell proceeds much 

 further. Some take merely the office of nutrition ; others 

 that of reproduction ; a third group constitute the outer 

 covering of the body and form the skin ; a fourth group, 

 the muscle cells, form the flesh ; a fifth group, the nerve 

 cells, develop into the organ of sensation, of will, of 

 thought, etc. All these different kinds of cells originally 

 proceeded by differentiation or specialization from the 

 single egg cell and from the homogeneous descendents of 



