Chap. VI. 
SUMMAEY. 
205 
simultaneously very different functions, and then having 
been specialised for one function ; and two very distinct 
organs having performed at the same time the same 
function, the one having been perfected whilst aided 
by the other, must often have largely facilitated 
transitions. 
We are far too ignorant, in almost every case, to be 
enabled to assert that any part or organ is so unim¬ 
portant for the welfare of a species, that modifications 
in its structure could not have been slowly accumulated 
by means of natural selection. But we may confidently 
believe that many modifications, wholly due to the laws 
of growth, and at first in no way advantageous to a spe¬ 
cies, have been subsequently taken advantage of by the 
still further modified descendants of this species. We 
may, also, believe that a part formerly of high import¬ 
ance has often been retained (as the tail of an aquatic 
animal by its terrestrial descendants), though it has 
become of such small importance that it could not, in 
its present state, have been acquired by natural selec¬ 
tion,—a power which acts solely by the preservation of 
profitable variations in the struggle for life. 
Natural selection will produce nothing in one species 
for the exclusive good or injury of another; though it 
may well produce parts, organs, and excretions highly 
useful or even indispensable, or highly injurious to 
another species, but in all cases at the same time useful 
to the owner. Natural selection in each well-stocked 
country, must act chiefly through the competition of 
the inhabitants one with another, and consequently will 
produce perfection, or strength in the battle for life, only 
according to the standard of that country. Hence the 
inhabitants of one country, generally the smaller one, 
will often yield, as we see they do yield, to the inha¬ 
bitants of another and generally larger country. For in 
