THE ELEMENTS OF ORGANIC EVOLUTION. 



8; 



ratio — that is, by multiplication. The increase of food 

 supply is by arithmetical ratio — that is, by addition ; 

 hence, whatever may be the ratio of increase, a geo- 

 metrical progression will sooner or later outrun an arith- 

 metical one. Hence, sooner or later the world must 

 be overstocked, did not vice, misery, or prudence come 

 in as checks, reducing the ratio of multiplication. This 

 law has been criticised as a partial truth, so far as man 

 is concerned. This means simply that there are factors 

 also in evolution other than those recognised by Mal- 

 thus. Nevertheless, Malthus's law is a sound statement 

 of one great factor. And this law is simply the ex- 

 pression of the struggle for existence as it appears 

 among men. 



In a world limited in extent and in possibilities, any 

 rate of increase among organisms must bring about a 

 struggle for existence. The ratio of increase is a mat- 

 ter of minor importance, for each species would fill up 

 the whole world at last. It is the ratio of actual net 

 increase above loss which determines the fate of a spe- 

 cies. Those increase and maintain themselves in which 

 the death rate does not exceed the rate of increase. 

 Those who live " beyond their means " must sooner or 

 later perish. 



Thus it comes about through natural selection that 

 there is everywhere seemingly perfect adaptation, the 

 " fitting of the dough to the pan," of the river to its bed. 

 But this fitting is never wholly perfect, for still more 

 complete adaptation may come ; and as conditions change 

 adaptations must change also. Progress follows organic 

 dissatisfaction. Where there is no reason for change 

 there is no progress; degeneration may set in, and de- 

 generation of one sort or another follows withdrawal 

 from the current of the struggle for existence. "What- 

 ever is desirable," says Weismann, " becomes necessary 



