EVOLUTION MADE PLAIN 23 
tions-by filling the world with its descendants 
if all of the one species were permitted to reach 
old age. If a pair of elephants (one of the 
slowest-breeding species of animals) should 
bring forth only six young ones, and all should 
live to be one hundred years old, their descend¬ 
ants in 750 years would number nineteen mil¬ 
lion—a number so great that they would form a 
closely-packed herd occupying forty-one square 
miles. The codfish produces nine million eggs 
a year. If each egg should develop into a ma¬ 
ture fish, half of them females, in ten years 
the sea would be a solid mass of codfish. 
This tendency to increase at so tremendous 
a rate, and the fact that no two individuals are 
exactly alike, supply the material and the con¬ 
ditions for the great law of natural selection 
to operate. Each individual must of necessity 
compete so fiercely with other individuals of 
the same species, and of allied species, and 
with enemies, with climate and changing con¬ 
ditions that out of the struggling many only 
a few live to propagate. Those individuals 
that vary from the mass in the right direction 
live; the others die, leaving few descendants, 
or none. 
Though like produces like there are no two 
individuals exactly alike. If all of the same 
species were alike there would be no opportu¬ 
nity for the law of natural selection to operate. 
Nothing but chance would determine in the 
struggle for existence which individuals would 
live to propagate their species and which would 
die without descendants. Nor would it matter, 
so far as the species is concerned. But indi¬ 
viduals do differ in their traits in some degree 
