GREAT STEPS IN ORGANIC EVOLUTION 391 



thousands of cells. The establishment of a body was one 

 of the mysterious big lifts in evolution, rising to a new 

 grade of organisation. More scope was given for specialisa- 

 tion of function, and indirectly for increase in size, since 

 free-living single cells cannot grow large except in very 

 peaceful surroundings. Getting a body made great increase 

 in size possible, which, other things equal, counts for some- 

 thing in a rough and callous physical environment, especially 

 if anything aggressive is to be done. One advantage is the 

 possibility of storing energy for vigorous assault on the 

 environment. Another advantage is the possibility of resting 

 and of lying low. Capitalisation has always meant much 

 in evolution. 



But the nemesis of gaining a body was liability to natural 

 death, — a liability proportionate to the complexity of the 

 bodily framework. For the more differentiation there is 

 in the colloid substratum in which the chemical processes 

 take place, the more difficult is it for processes of repair 

 and rejuvenescence to counteract that accumulation of wear 

 and tear results which spells senescence (see Child, Senes- 

 cence and Rejuvenescence, Chicago, 1915). 



§ 6. The Divergence of the Sexes. 



Another step with far-reaching consequences was the evo- 

 lution of male and female multicellular individuals within 

 the same species, the two being complementary in the con- 

 tinuance of the race. The first hints of this were among 

 the Protozoa, but these are probably on a side-track. The 

 big fact was the origin of two dimorphic types within the 

 species, a dichotomy like that between plants and animals, 

 probably expressing alternative rates or rhythms of biochem- 

 ical routine, and culminating in the contrasts between pea- 



