182 THE STUDY OF ANIMAL LIFE CHAP. 



tion, and in being very definitely a regulated process. 

 But growth is not to be thought of as an end or accom- 

 plishment in itself. Size as such counts for little, and 

 the dwarf bends the Titan to his will. Growth is impor- 

 tant because it means capital, resources, reserves ; it 

 makes trading, experimenting, adventure more possible ; 

 it facilitates differentiation on the one hand, and mastery 

 of the medium on the other. To take a very obvious 

 illustration, there are not a few animals, like Planarians, 

 that can live for months on their own resources not 

 on nutritive reserves, but literally on themselves. But 

 it was previous growth that made this possible. 



(b) But growth leads on to multiplication. For, as 

 Haeckel was one of the first to state clearly (in his Gene- 

 relle Morphologic, 1866), reproduction is discontinuous 

 growth. Is it possible to draw any hard-and-fast line 

 between a fragmentation which separates off over- 

 growths and the more specialised forms of reproduction ? 

 It is the growth of a cell that leads to its division, though 

 we do not know what the internal instabilities are that 

 bring about the mysterious process. Spencer, Leuckart, 

 and James pointed out independently that as a cell of 

 regular shape increases in volume it docs not propor- 

 tionately increase in surface. If it be a sphere, the volume 

 of material to be kept alive increases as the cube of the 

 radius, while the surface, through which the keeping- 

 alive is effected, increases only as the square. Thus 

 there tends to be a hazardous disproportion between 

 volume and surface, which may set up instability. The 

 disturbed balance is normally restored by the cell divid- 

 ing into two cells. This only leads us a little way towards 

 understanding cell-division, but it seems clear that the 

 correlation of chemical processes in a colloidal substratum 

 which makes continued self-maintenance possible natur- 

 ally leads on to growth, and that growth naturally leads 

 on to division or reproduction. In any case there is 

 nothing more distinctive of living organisms than this 

 power of giving origin to new lives. 



(c) It is possible, however, to take another step, logi- 

 cally connected with multiplication, just as multiplica- 



