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.
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