REPRODUCTION AND SEX 457 



duction is discontinuous growth" remains profoundly true, it needs 

 further elaboration, at least in the higher organisms, if not perhaps 

 in all. 



The importance of the relation between reproduction and the 

 limit of growth may be illustrated in reference to the cell, or to 

 any compact one-celled organism, such as a spherical Sporozoon or 

 an ovoid yeast-plant. When a growing cell (or unicellular organism) 

 of regular shape, spherical let us say, increases its volume several 

 times, its surface does not increase at the same rate; volume in- 

 creases as the cube of the radius, and surface only as the square. 

 This disproportion is very important, for it is by the surface that 

 the living matter of the cell is fed, aerated, and purified, in short, 

 kept alive. If the cell goes on growing beyond the limit at which the 

 surface can meet the requirements of the volume of living matter, 

 then instability is inevitable. The regulated way of dealing with this 

 is for the cell to divide into two, thus increasing surfaces in ratio of 

 volumes, and towards growth anew. Thus it is that the reproduction 

 of the cell occurs at the limit of growth. No doubt other factors 

 have to be considered besides the ratio of surface and volume ; thus 

 there is the relation between the nucleus of the cell and the general 

 cell-substance — the ratio of nucleoplasm to cytoplasm — so much 

 emphasised by R. Hertwig. There is as yet too little understanding 

 of the precise nature of the instability that directly induces the cell- 

 division; hence the hypotheses implied in terms like "auto-toxic", 

 "enzymatic", cytolytic — each expressing an aspect of the process. 

 There is also difficulty in passing from the consequences of the limit 

 of growth in a single cell to those that apply in the physiological 

 economy of a large animal. Enough here that study of the hmit of 

 growth in cells and cell-aggregates throws some light on the physio- 

 logical necessity for reproduction. As we have noticed in another 

 section, one of the great trends of evolution is towards increase 

 of internal surface, and cell-division is one of the deepest expressions 

 of this. 



Nutrition and Reproduction. — Another general idea to be 

 kept in mind is the physiological antithesis between nutrition and 

 reproduction. In conditions of abundant food the common Hydra 

 produces many adherent buds, literal reproductions of itself. A bud, 

 while still attached to the parent, may bear buds of its own. Eventu- 

 ally, a check to nutrition occurs; conditions set in which are not 

 favourable to continued assimilation; the buds separate off, and 

 this may be followed by a phase of sexual reproduction. 



Similarly a Planarian worm in good nutritive conditions may 

 form asexually a chain of four daughter- worms ; a check to nutrition 

 may occur; the links separate; and after a varying period of free 

 Hfe sexual reproduction may set in. There are many illustrations 

 of this organic see-saw between nutrition and reproduction. Vigor- 



