240 EN TOMOLOGY 
larvae, parasitized by Braconidee and Chalcidide, carried off 
by some of the digger-wasps (Mimeside, Pemphredonide), 
and devoured by ants, carabids, other insects, spiders, and some 
birds, as the chickadee. In damp weather, aphids are killed 
in countless numbers by a fungous disease. In short, the 
aphid is threatened in every direction. 
Elimination of the Unfit.—In the intense 
existence,” as it is commonly, though misleadingly, called, 
“struggle for 
those comparatively few individuals that survive do so mani- 
festly by virtue of certain advantages over their less fortunate 
fellows. One egg can stand a little more cold than another; 
one beetle drops to the ground when disturbed and thus 
escapes an attacking bird, while its companions remain in place 
and are destroyed; some individuals escape by surpassing their 
fellows in locomotor ability or by resembling the surface on 
which they happen to rest. : 
Such fortunate individuals live to transmit their advantage- 
ous peculiarities to their progeny, while the less favored indi- 
viduals succumb. The progeny inherit the life-saving pecu- 
liarities in differing degrees, and the least favored of the 
progeny are again weeded out. Thus by the continual elim- 
ination of individuals that vary in unfavorable directions, the 
individuals that remain become better and better adapted to 
the surrounding conditions of life, through the preservation 
and accumulation of advantageous variations. ‘This preser- 
vation and accumulation of advantageous variations through 
the destruction of disadvantageous ones is the essence of nat- 
ural selection, or the ‘‘ survival of the fittest.” 
Favorable variations may have been so slight and infre- 
quent as to have required geological ages for their accumula- 
tion. On the other hand, adaptive variations are sometimes 
so extensive from the beginning as to lead some writers to 
doubt that these variations are preserved and improved by 
natural selection. 
Variation.— Natural selection cannot originate useful char- 
acters, of course, but is limited to the preservation and accu- 
