110 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 
