t different habits of life could not graduate into each
other; that a bat, for instance, could not have been formed by natural
selection from an animal which at first only glided through the air.

We have seen that a species under new conditions of life may change its
habits, or it may have diversified habits, with some very unlike those
of its nearest congeners. Hence we can understand, bearing in mind that
each organic being is trying to live wherever it can live, how it has
arisen that there are upland geese with webbed feet, ground woodpeckers,
diving thrushes, and petrels with the habits of auks.

Although the belief that an organ so perfect as the eye could have been
formed by natural selection, is enough to stagger any one; yet in
the case of any organ, if we know of a long series of gradations in
complexity, each good for its possessor, then under changing conditions
of life, there is no logical impossibility in the acquirement of any
conceivable degree of perfection through natural selection. In the cases
in which we know of no intermediate or transitional states, we should
be extremely cautious in concluding that none can have existed, for the
metamorphoses of many organs show what wonderful changes in function
are at least possible. For instance, a swim-bladder has apparently been
converted into an air-breathing lung. The same organ having performed
simultaneously very different functions, and then having been in part
or in whole specialised 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