members of
a class, for in this latter case the organ must have been originally
formed at a remote period, since which all the many members of the class
have been developed; and in order to discover the early transitional
grades through which the organ has passed, we should have to look to
very ancient ancestral forms, long since become extinct.

We should be extremely cautious in concluding that an organ could not
have been formed by transitional gradations of some kind. Numerous cases
could be given among the lower animals of the same organ performing
at the same time wholly distinct functions; thus in the larva of the
dragon-fly and in the fish Cobites the alimentary canal respires,
digests, and excretes. In the Hydra, the animal may be turned inside
out, and the exterior surface will then digest and the stomach respire.
In such cases natural selection might specialise, if any advantage
were thus gained, the whole or part of an organ, which had previously
performed two functions, for one function alone, and thus by insensible
steps greatly change its nature. Many plants are known which regularly
produce at the same time differently constructed flowers; and if such
plants were to produce one kind alone, a great change would be effected
with comparative suddenness in the character of the species. It is,
however, probable that the two sorts of flowers borne by the same plant
were originally differentiated by finely graduated steps, which may
still be followed in some few cases.

Again, two distinct organs, or the same organ under two very different
forms, may simultaneously perform in the same individual the same
function, and this is an extremely important means of transition: to
give one instance--there are fish with gills or branchiae that breathe
the air dissolved in the water, at the same time that they breathe free
air in their swim-bladders, this latter organ being divided by highly
vascular partitions and having a ductus pneumaticus for the supply of
air. To give another instance from the vegetable kingdom: plants climb
by three distinct means, by spirally twining, by clasping a support with
their sensitive tendrils, and by the emission of aerial rootlets; these
three means are usually found in distinct groups, but some few species
exhibit two of the means, or even all three, combined in the same
individual. In all such cases one of the two organs might readily be
modified and perfected so as to perform all th