204 



THE EVOLUTION OE SEX. 



few of the Protozoa, such as the large amoeboid Pelomyxa, some of the 

 gregarines, and even more markedly the extinct nummulites, which 

 were sometimes as large as half-crowns. So an occasional alga, like 

 Botryduan. may swell out into a large single cell, and the ova of 

 animals, for example, birds, are often greatly expanded by the accu- 



oOf) 



Fig. 77. — Cell-division at the limit of growth. 



mulation of yelk. Yet the unit-masses generally remain very small. 

 They have their maximum size, approximately constant for each 

 species. Up to this point they grow, but no further. The same, as 

 every one knows, is true of multicellular animals. The size fluctuates 

 slightly according to the conditions of individual life, but the average 

 is strikingly constant. 



II. Spencer's Theory of Growth. — The first adequate discus- 

 sion of growth is due to Spencer. He pointed out that in the growth 

 of similarly shaped bodies the increase of volume continually tends to 

 outrun that of the surface. The mass of living matter must grow 

 more rapidly than the surface through which it is kept alive. In 

 spherical and all other regular units the mass increases as the cube of 

 the diameter, the surface only as the square. Thus the cell, as it 

 grows, must get into physiological difficulties, for the nutritive neces- 

 sities of the increasing mass are ever less adequately supplied by the 

 less rapidly increasing absorbent surface. The early excess of repair 

 over waste secures the growth of the cell. Then a nemesis of growing 

 wealth begins. The increase of surface is necessarily disproportionate 

 to that of contents, and so there is less opportunity for nutrition, 

 respiration, and excretion. Waste thus gains upon, overtakes, 

 balances, and threatens to exceed repair. Suppose a cell to have 

 become as big as it can well be, a number of alternatives are possible. 

 Growth may cease, and a balance be struck ; or the form of the unit 

 may be altered, and surface gained by flattening out, or very frequently 

 by outflowing processes. On the other hand, waste may continue on 

 the increase, and bring about dissolution or death ; while closeiy akin 

 to this, there is the most frequent alternative, that the cell divide 



