240 ENTOMOLOGY 



larvae, parasitized by Braconidae and Chalcididae, carried off 

 by some of the digger-wasps (Mimesidae, Pemphredonidae), 

 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 " struggle for 

 existence," as it is commonly, though misleadingly, called, 

 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- 



