24 EVOLUTION MADE PLAIN 
—Wallace has shown that variation usually 
amounts from ten to twenty, sometimes twenty- 
five, per cent of the varying part—and this 
variation, even if small, often spells the differ¬ 
ence between an early death with no descend¬ 
ants and a long life with a numerous progeny. 
By the law of heredity the descendants are 
endowed in a greater or less degree with the 
same life-saving characteristics of their par¬ 
ents. Each generation being subjected to this 
weeding-out process, and only a few of the best 
fitted individuals being selected to preserve 
the species, we can easily see that, as the gen¬ 
erations come and go, those essential, life-sav¬ 
ing characters or traits are being developed 
to a greater and greater degree. And this 
means change, modification of species. 
With horn or tooth or claw or hoof or sting 
or poison or odor, among all the wild creatures 
that swim or crawl or run or climb or fly, the 
struggle for life and food and mate goes on 
today just as it has gone on for millions of 
years. Other factors in the struggle are alert¬ 
ness, agility, cunningness, sharpness of vision 
and hearing and smell, protective color of cov¬ 
ering, fleetness of foot and wing, the degree of 
heat, the amount of moisture and the food 
supply. 
The traits that survive are of course those 
that are the most useful in a given environ¬ 
ment. (By environment is meant surroundings, 
all outside influences, the not-me of each in¬ 
dividual.) A change of environment, of the 
conditions of life, calls for a re-adjustment of 
the traits most vital to the individual in the 
new environment. In one environment one 
