ous. White parts of animals arc often attacked with disease, while the other parts remain healthy; and light-skinned animals are most troubled by flies and parasites. Albinoism, which is simply a total inaptitude for the production of pigment, is a sure sign of degeneracy.
Vigor of the genital organs is one of the most manifest signs of vital activity. The relation between the reproductive function and pigmentation is so striking that Heusinger has expressed it as a law. Troubles brought upon the sexual functions under the influence of any particular causes, as of domestication, often coincide with the most singular modifications of color.
The coloring-matter is also intimately connected with the nervous system. Thus, it is at the extremity of a nerve, the optic nerve, that is localized, in all species of animals, the maximum of aptitude for the production of pigment. In the lowest types of the series, when the eye begins to become differentiated, and while it can hardly yet be considered an organ of vision, a pigment-spot may be observed to make its appearance. At the same time other parts of the optical apparatus that have a much greater functional importance, the refracting media, for example, may not yet be existing even in a rudimentary state. These considerations lead me to believe that the optical pigment-spot owes its existence not solely to the advantages which the individual may derive from it, but chiefly to the proximity of a nerve, the elements of which are disturbed by a continuous vibrating movement, or by light. This kind of election of pigment exists, moreover, not only in reference to the organ of sight, but frequently also in other special sensitive terminations—at the ends of the auditory nerves with some invertebrates, at the end of the proboscis in the Nemertes. In the chameleon, the turbot, the cuttle-fish, and some other animals, the connection of the pigmentary system with the nerves is so close that a simple nervous excitation is enough to modify the distribution of the colored granulations in the integuments.
On the other hand, certain constitutional defects induce a diminution or absence of coloring-matter, of which I can give no better illustration than to cite Darwin's curious observation that white cats generally have blue eyes and are deaf.
What we have said tends to prove that the positive facts of life, or the complete development of the organs of the individual—health, strength, fullness of functions, display of activity and accentuation of animal vigor in the nervous system and the organs of relation—correspond closely with an abundant production of coloring-matter; while the negative facts of life—age, constitutional weakness, disease, apathy, and degeneration generally—lead to a more or less complete disappearance of the same substance. Nevertheless, we notice in some cases the contrary fact, or a deposition of coloring-matter, or an increase of its production in connection with some pathological condition of the organism. But these cases, which seem opposed to our theory,