286 DISEASE IN I'l.ANTS. 



Up (assimilation) and shatterings (dis-assimilation) 

 of the protoplasm molecules are the growth of the 

 protoplasm, with further intercalations of water 

 and new food-supplies, etc., on the one hand, and 

 the formation of metabolic products (proteids, 

 cellulose, sugars, fats, etc.), some of which are 

 again used up, others respired, others deposited as 

 stores, cell-walls, etc., on the other. 



That the building-up process depends on the 

 action of molecular forces comparable to those by 

 which a growing crystal goes on selecting atom- 

 complexes of its particular kind from the solution 

 around seems highly probable, and this being the 

 case we can understand how under certain cir- 

 cumstances substitutive selections may occur. 

 That is to say, just as a crystal will sometimes 

 build up into its structure atom-complexes of a 

 kind different from its normal molecules, so, given 

 the proper conditions, a protoplasmic molecular 

 unit will build up into its structure atom-complexes 

 somewhat different from those it had hitherto 

 taken up i.e. assimilated with consequent 

 modifications of its behaviour. If this occurs, the 

 modes of further building up and breaking down 

 will be affected by the subsequent action of these 

 slightly modified protoplasm units, and it may 

 ivell be that the whole significance of variation 

 turns on this. Whether the resulting variation 

 makes for the welfare or otherwise of the 

 organism will then be decided by the struggle 

 for existence, and the natural selection which 

 ensues. Such a view also implies that the 



