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. If a 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 



