or a special principle of explanation; for hair and
teeth are similar formations,[31] and the same chemical change of the
germ that hinders the formation of hair would probably obstruct that of
teeth: it may be for the same sort of reason that white cats with blue
eyes are deaf. In these different examples the "correlative" changes are
only _solidary_ changes (not to mention the fact that they are really
_lesions_, namely, diminutions or suppressions, and not additions, which
makes a great difference). But when we speak of "correlative" changes
occurring suddenly in the different parts of the eye, we use the word in
an entirely new sense: this time there is a whole set of changes not
only simultaneous, not only bound together by community of origin, but
so coordinated that the organ keeps on performing the same simple
function, and even performs it better. That a change in the germ, which
influences the formation of the retina, may affect at the same time also
the formation of the cornea, the iris, the lens, the visual centres,
etc., I admit, if necessary, although they are formations that differ
much more from one another in their original nature than do probably
hair and teeth. But that all these simultaneous changes should occur in
such a way as to improve or even merely maintain vision, this is what,
in the hypothesis of sudden variation, I cannot admit, unless a
mysterious principle is to come in, whose duty it is to watch over the
interest of the function. But this would be to give up the idea of
"accidental" variation. In reality, these two senses of the word
"correlation" are often interchanged in the mind of the biologist, just
like the two senses of the word "adaptation." And the confusion is
almost legitimate in botany, that science in which the theory of the
formation of species by sudden variation rests on the firmest
experimental basis. In vegetables, function is far less narrowly bound
to form than in animals. Even profound morphological differences, such
as a change in the form of leaves, have no appreciable influence on the
exercise of function, and so do not require a whole system of
complementary changes for the plant to remain fit to survive. But it is
not so in the animal, especially in the case of an organ like the eye, a
very complex structure and very delicate function. Here it is impossible
to identify changes that are simply solidary with changes which are also
complementary. The two senses of th