THE DARWINIAN THEORY 



551 



The Struggle for Existence. — As a result of these conditions of 

 overproduction and stability, many more individuals come into 

 existence and struggle to live than can possibly survive. Hence, 

 the conclusion is drawn that there is a " struggle for existence " 

 in all hfe. This phrase was used by Darwin in a general and meta- 

 phorical sense, for the struggle is seldom an actual conflict; al- 



a^ 



Fig. 306. — The lady beetle, Pentilia misella, and its larvae feeding upon 

 another insect, the San Jose scale, a dangerous enemy of fruit trees. 



Various species of lady beetles have proved efficient checks upon the multiplication 

 of scale insects, a, adult; b, larva; c, pupa; d, the beetles and larvae feeding upon the 

 scale insects, the scale-like bodies of which are shown upon a twig of a pear tree. (From 

 Bulletin No. 3, N. S., U. S. Dept. Agr. Div. Entomology.) 



though it may be so, as when carnivorous animals fight with one 

 another for food or mates. In general, it is most acute between 

 individuals of the same species, since these compete for the same 

 conditions of life. A similar competition may occur between 

 widely different species, if they use the same food, as when insects 

 devour the food of grazing animals. Another example of struggle 

 between individuals of different species is that of carnivorous forms 



