offspring. Here the function is certain whatever view we take of the
origin of the organs, whether we believe they were created or evolved. But
if we consider the flipper or paddle of a whale, we see that it is
homologous with the fore-leg of a terrestrial mammal, and we are in the
habit of saying that in the whale the fore-limb is modified into a paddle
and has become adapted for aquatic locomotion. This, of course, assumes
that it has become so adapted in the course of descent. But the pectoral
fin of a fish is equally 'adapted' for aquatic locomotion, but it is
certainly not the fore-leg of a terrestrial mammal adapted for that
purpose. The original meaning of adaptation in animals and plants, of
organic adaptation to use another term, is the relation of a mechanism to
its action or of a tool to its work. A hammer is an adaptation for
knocking in nails, and the woodpecker uses its head and beak in a similar
way for making a hole in the bark of trees. The wings and the whole
structure of a bird's body form a mechanism for producing one of the most
difficult of mechanical results, namely, flight. Then, again, there are
stationary conditions, such as colour and patterns, or scales and armour,
which may he useful in the life of an animal or flower, but are not
mechanisms of moving parts like a bird's wing, or secreting organs like
mammary glands. Unless we choose or invent some new term, we must define
adaptations apart from all questions of evolution as any structures or
characters in an organism which can be shown either by their mere
presence, or by their active function, to be either useful or necessary to
the animal's existence. We must be on our guard against assuming that the
word 'adaptation' implies any particular theory or conclusion concerning
the method and process by which adaptations have arisen in the course of
evolution. It is that method and process which we have to investigate.

On the other hand, when we look primarily at differences of structure we
find that not only are there wide and distinct gaps between the larger