Distribution by Sex 359 



in the new phase, cells associate in mutual support, with 

 specialization of function which is not lost, but is, by sexual 

 heredity, transmissible and cumulative. The degree of asso- 

 ciation of single cells may at first amount to nothing more 

 than tolerance, and may be so slight that a little crowding 

 upsets and destroys it. On the other hand it may soon be- 

 come so close and so habitual that it is necessary for exis- 

 tence. Examples of both habits are found. There are to 

 this day single-celled creatures making no sign of associated 

 action, beyond the mere submission to propinquity, which 

 becomes in due time unendurable ; and there are others whose 

 habit of crowding is so necessary that it is difficult to recog- 

 nize their single-celled existence. There are vegetable forms 

 which show inherited specialization, preliminary to the 

 greater advance. The direction of the great step is deter- 

 mined when a number of cells in close association, become 

 fitted for different duties, still having a common purpose, 

 and then by habit become settled in those duties, and by 

 heredity confirmed in them, so that none others are de- 

 manded of them. Let us imagine that a cell just divided or 

 budded from a parent cell, should, for some reason, remain 

 in contact with it. Perhaps because it afforded more sta- 

 bility to a form floating in water, or resting on the bottom 

 of a water pool; a few creatures in which such contact arose 

 by chance, might be so benefited as to evolve a new variety. 

 Perhaps there is a yet undiscovered system by which such 

 advantageous variations are stimulated, instead of being 

 merely survivals of accident. In either case we have, in fact, 

 two-celled creatures. That is to say creatures composed of 

 one prime cell and another associated cell. This conception 



