ORIGIN OF ADAPTATIONS AND OF SPECIES 239 
increase is unchecked, the number of individuals of the twelfth 
generation, as the descendants of a single female of the first 
generation, would be ten sextillions. These if placed in a 
single file, allowing 10 aphids to an inch, would form a line so 
long that light itself, traveling at the rate of 186,000 miles 
per second, would require over 2,690 years to go from one 
end of the line to the other. 
As it is, many species become temporarily dominant under 
favorable conditions; for example, the Rocky Mountain locust, 
chinch bug and gypsy moth. Even one of the least prolific 
species would predominate in a surprisingly short time, were 
it permitted to increase in its normal geometrical ratio. The 
rate of sexual reproduction is highest in fishes and insects. An 
insect averages one or two hundred eggs, while some forms, 
as queen termites, lay them by thousands. 
Struggle for Existence.—Although a single species is 
potentially capable of covering the earth, there actually are at 
least 1,000,000 species of insects, not to mention 250,000 spe- 
cies of other animals and some 500,000 kinds of plants. This 
means a tremendous prevention of reproduction among the 
individuals of any one species—an intense “ struggle for ex- 
istence,” as Darwin termed it. Among plants and the lower 
animals, comparatively few individuals survive and reproduce; 
the majority die. The agents of destruction are manifold, 
each species having its own army of enemies, organic and in- 
organic. ‘Thus insects are subject to unfavorable conditions 
of temperature and moisture, to bacterial and fungous dis- 
eases, vertebrate and invertebrate enemies, accidents, etc. 
The aphids are at the same time among the most prolific and 
the most defenceless of animals. ‘These delicate insects suc- 
cumb to very slight mechanical shocks and are killed by ex- 
tremes of temperature that most other insects can endure. 
They are often washed off their food plants by rain. Their 
rate of reproduction decreases if their food plant receives in- 
sufficient moisture. Aphids form the chief food of coccinellid 
larvee and beetles, are preyed upon by chrysopid and syrphid 
