similar to itself; its digestive organs must be adapted to the kind of
food it takes; it must have respiratory organs adapted to breathe in air
or water; it must produce eggs able to survive in particular conditions,
and so on.

One of the most interesting results of the study of the facts of evolution
is that each type of animal tends to multiply to such an extent as to
occupy the whole earth and adapt itself to all possible conditions. In the
Secondary period reptiles so adapted themselves: there were oceanic
reptiles, flying reptiles, herbivorous reptiles, carnivorous reptiles. At
the present day the Chelonia alone include oceanic, fresh-water, and
terrestrial forms. Birds again have adapted themselves to oceanic
conditions, to forests, plains, deserts, fresh waters. Mammals have
repeated the process. The organs of locomotion in such cases show profound
modifications, adapting them to their special functions. One thing to be
explained is the origin of adaptations.

It is, however, necessary to distinguish between the adapted condition or
structure of an organ and the process by which it became adapted in
evolution; two ideas which are often confused. The eye would he equally
adapted for seeing whether it had been created in its actual condition or
gradually evolved. We have to distinguish here, as in other matters,
between being and becoming, and, further, to distinguish between two kinds
of becoming--namely, the development of the organ in the individual and
its evolution in the course of descent. The word 'adaptation' is itself
the cause of much fallacious reasoning and confusion of ideas, inasmuch as
it suggests a process rather than a condition, and by biological writers
is often used at one time to mean the former and at others the latter. We
may take the mammary glands of mammals or organs adapted for the secretion
of milk, whose only function is obviously the nourishment of the