VARIATION BY USE AND DISUSE. 
255 
intense between individuals of the same species—as 
sparrow against sparrow, wolf against wolf, beetle 
against beetle. They require the same food, and 
Fig. 118.—The cottony cushion- 
scale insect of California. The 
male is winged, the female 
wingless and with a large 
waxen egg-sac attached to her 
body. (The lines at the left 
of each figure indicate the 
size of the insects.) (Jordan 
and Kellogg’s Animal Life.) 
-e s 
when the species is greatly increased, the food supply 
is insufficient, and the weaker ones are forced to the 
wall. Then there is a struggle between related species 
—as sparrow against robin, beech against birch. 
Species which live upon entirely different food do 
