THE INDIVIDUAL AND THE SPECIES 169 



would always be a definite function of the length of 

 this side. The mass of a cube would also be a function 

 of such a measurement : it would be al z , a being a 

 constant depending on the unit of mass and on the 

 specific weight of the substance of which the crystal 

 was composed. If we take a series of crystals of 

 increasing size, this relation holds for every one of 

 them : M=al 3 , M being the mass, a the constant 

 referred to above, and /, the independent variable, 

 being any one length of a side of the crystal. 



If the organism grows by accretion in the same way 

 as does a crystal, this relation ought also to hold in all 

 the exclusiveness with which we expect it to hold in 

 the growth of a crystal. But it does not so grow. Its 

 growth is something essentially different, and none 

 of the superficial analogies so prevalent nowadays 

 ought to obscure this difference. The organism may 

 grow by accretion, thus layers of calcareous matter 

 may be added to the outside of a membrane bone from 

 the investing periosteum, or it may grow by the 

 deposition of matter within the actual cell bodies, 

 (the process of growth by intussusception of the plant 

 physiologists). But the extent of growth by accretion 

 is strictly limited in all organisms : for each there is 

 a maximal mass determined by the nature of the 

 animal or plant, and this mass is that of the uni- 

 cellular organism itself, or that of the cells of which 

 the multi-cellular organism is composed. There may 

 also be growth by accretion in the case of the formation 

 of skeletal structures, which are laid down by the 

 agency of the cells of the organism ; but if we confine 

 our attention to the growth of the actual living sub- 

 stance we shall see that accretion ceases when the 

 mass characteristic of the cells has been attained, 

 when growth by dissociation begins. The cell then 



