120 ‘ ANIMAL LIFE 
demands and survive, the rest die. The survivors produce 
young similarly advantageously different from the general 
type, and the adaptation increases with successive genera- 
tions. 
71. Adjustment to surroundings a result of natural selec- 
tion.—To such causes as these we must ascribe the nice 
adjustment of each species to its surroundings. Ifa species 
or a group of individuals can not adapt itself to its environ- 
ment, it will be crowded out by others that can do so. The 
former will disappear entirely from the earth, or else will be 
limited to surroundings with which it comes into perfect 
adjustment. A partial adjustment must with time become 
a complete one, for the individuals not adapted will be 
exterminated in the struggle for life. In this regard very 
small variations may lead to great results. A side issue 
apparently of little consequence may determine the fate of 
a species. Any advantage, no matter how small, will turn 
the scale of life in favor of its possessor and his progeny. 
“ Battle within battle,” says a famous naturalist, “must be 
continually recurring, with varying success. Yet in the 
long run the forces are so nicely balanced that the face of 
Nature remains for a long time uniform, though assuredly 
the merest trifle would give the victory to one organic being 
over another.” 
72. Artificial selection.—It has been long known that the 
nature of a herd or race of animals can be materially altered 
by a conscious selection on the part of man of these indi- 
viduals which are to become parents. To “weed out” a 
herd artificially is to improve its blood. To select for re- 
production the swiftest horses, the best milk cows, the most 
intelligent dogs, is to raise the standard of the herd or 
race in each of these respects by the simple action of hered- 
ity. Artificial selection has been called the “ magician’s 
wand,” by which the breeder can summon up whatever 
animal form he will. If the parentage is chosen to a defi- 
nite end, the process of heredity will develop the form 
