1 1 o Darwin, and after Darwin. 



conform to the economic principle of the division of 

 labour a principle, indeed, which is already fore- 

 shadowed in the constituent parts of a single cell, 

 since the nucleus has one set of functions and its 

 surrounding protoplasm another. 



But now, in the third place, we arrive at a more 

 important distinction, and one which lies at the root 

 of the others still remaining to be considered. I refer 

 to sexual propagation. For it is a peculiarity of the 

 multicellular organisms that, although many of them 

 may likewise propagate themselves by other means 

 (Fig. 28), they all propagate themselves by means 

 of sexual congress. Now, in its essence, sexual con- 

 gress consists in the fusion of two specialized cells 

 (or. as now seems almost certain, of the nuclei thereof), 

 so that it is out of such a combination that the new 

 individual arises by means of successive cell-divisions, 

 which, beginning in the fertilized ovum, eventually 

 build up all the tissues and organs of the body. 



This process clearly indicates very high specializa- 

 tion on the part of germ-cells. For we see by it that 

 although these cells when young resemble all other 

 cells in being capable of self-multiplication by binary 

 division (thus reproducing cells exactly like them- 

 selves), when older they lose this power ; but, at 

 the same time, they acquire an entirely new and very 

 remarkable power of giving rise to a vast succession 

 of many different kinds of cells, all of which are 

 mutually correlated as to their several functions, so 

 as to constitute a hierarchy of cells or,' to speak 

 literally, a multicellular co-organization. Here it is 

 that we touch the really important distinction between 

 the Protozoa and the Metazoa ; for although I have 



