THE ELEMENTS OF ORGANIC EVOLUTION. 87 
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 changealso. 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 
