l62 THE EVOLUTION OF MAH. 



labour occurs in those lower animals in the bodies of which 

 only two kinds of cells have become differentiated. This 

 is the case, for example, in the lowest Plant-animals, in 

 Sponges, and the simplest Polyps, as well as in their 

 common parent-form, the Gastrsea. Throughout the entire 

 many-celled bodies of these, there are only two different 

 kinds of cells ; the one kind effect the nutrition and repro- 

 duction 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 embryo. 

 But in most higher animals the differentiation of the cells 

 proceeds much further. Some take merely the office of 

 nutrition ; others that of reproduction ; a third group con- 

 stitute the outward 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 organs of 

 sensation, of will, of thought, etc. All these different kinds 

 of cells originally proceeded by differentiation or specialisa- 

 tion from the simple egg-cell, and from the homogeneous 

 descendants of that egg-cell, owing to division of labour. 

 This differentiation of the cells, or this division of labour, 

 originally arose in tribal history, from causes similar to the 

 division of labour in the civilized states of men. Afterwards 

 it appears in the germ-history, and by that time it has been 

 made over to Heredity, and is merely repeated in accord- 

 ance with the fundamental law of Biogeny. Now, although 

 Differentiation usually leads to the progress of the whole 

 organism as well as of its various constituent individuals, 

 the single cells, yet it is also in many cases the occasion of 

 retrogression, or atavism. Not only progressive, but also 



