106 Darwin, and after Darwin. 
have aptly called a process of discontinuous growth, 
That is to say, when a cell reaches maturity, further 
growth takes place in the direction of a severance of 
its substance—the separated portion thus starting 
anew as a distinct physiological unit. But, notwith- 
standing the complex changes which have been more 
recently observed to take place in the nucleus of some 
Protozoa prior to their div'sion, the process of 
multiplication by division may still be regarded as a 
process of growth. which differs from the previous 
growth of the individual cell in being attended by a 
severance of continuity. If we take a suspended drop 
of gum, and gradually add to its size by allowing 
more and more gum to flow into it, a point will 
eventually be reached at which the force of gravity 
will overcome that of cohesion, and a portion of the 
drop will fail away from the remainder. Here we 
have a rough physical simile, although of course no 
true analogy. In virtue of a continuous assimilation 
of nutriment, the protoplasm of a cell increases in 
mass, until it reaches the size at which the forces of 
disruption overcome those of cohesion—or, in other 
words, the point at which increase of size is no longer 
compatible with continuity of substance. Neverthe- 
less, it must not be supposed that the process is thus 
merely a physical one. The phenomena which occur 
even in the simplest—or so-called “direct ’—cell- 
division, are of themselves enough to prove that the 
process is vital, or physiological ; and this in a high 
degree of specialization. But so, likewise, are all 
processes of growth in organic structures ; and there- 
fore the simile of the drop of gum is not to be 
regarded as a true analogy: it serves only to 
