io6 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 division, 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 fall 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 



