REPRODUCTION. 645 



occurs, one more like that by which our western lands were 

 settled and gradually built up into Territories and States. 

 The new individual in the political world began with little 

 differentiation; it consisted of units, separated from older and 

 highly organized societies, and these units at first did pretty 

 much everything, each man for himself, with more or less 

 efficiency. As growth took place development also occurred; 

 persons assumed different duties and performed different 

 work until, finally, a fully organized State was formed. 

 Similarly, the body of one of the higher animals is, at an 

 early stage of life, merely a collection of undiiferentiated 

 cells, each capable of multiplication by division, and more or 

 less retaining all its original protoplasmic properties; and 

 with no specific individual endowment or function. The 

 mass (Chap. III.) then slowly differentiates into the various 

 tissues, each with a predominant character and duty; at the 

 same time the majority of the cells lose their primitive powers 

 of reproduction, though exactly how completely is a problem 

 not yet sufficiently studied. In adult Vertebrates it seems 

 certain that the white blood corpuscles multiply by division: 

 and in some cases (in the newts or tritons, for example) a 

 limb is reproduced after amputation. But exactly what cells 

 take part in such restorative processes is uncertain; we do 

 not know if the^ old bone corpuscles left form new bones, old 

 muscle-fibres new muscles, and so on; though it is probable 

 that the little-differentiated leucocytes build up most of the 

 new limb. In Mammals no such restoration occurs; an am- 

 putated leg may heal at the stump but does not form again. 

 In the healing processes the connective tissues play the main 

 part, as we might expect; their cellular elements being but 

 little modified from their primitive state (p. 102) can still 

 multiply and develop. New blood capillaries, however, sprout 

 out from the sides of old, and new epidermis seems only to be 

 formed by the multiplication of epidermic cells; hence the 

 practice, frequently adopted by surgeons, of transplanting 

 little bits of skin to points on the surface of an extensive 

 burn or ulcer. In blood capillaries and epidermis the de- 

 parture from the primary undifferentiated cell is but slight; 

 and, as regards the cuticle, one of the permanent physiologi- 

 cal characters of the cells of the rete mucosum is their multi- 

 plication throughout the whole of life; that is a main physio- 

 logical characteristic of the tissue: the same is very probably 



