for one function; and two 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 have seen that in two beings widely remote from each other in the
natural scale, organs serving for the same purpose and in external
appearance closely similar may have been separately and independently
formed; but when such organs are closely examined, essential differences in
their structure can almost always be detected; and this naturally follows
from the principle of natural selection. On the other hand, the common
rule throughout nature is infinite diversity of structure for gaining the
same end; and this again naturally follows from the same great principle.

In many cases we are far too ignorant to be enabled to assert that a part
or organ is so unimportant for the welfare of a species, that modifications
in its structure could not have been slowly accumulated by means of natural
selection. In many other cases, modifications are probably the direct
result of the laws of variation or of growth, independently of any good
having been thus gained. But even such structures have often, as we may
feel assured, been subsequently taken advantage of, and still further
modified, for the good of species under new conditions of life. We may,
also, believe that a part formerly of high importance has frequently 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 means of natural selection.

Natural selection can 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 possessor.