96 THE POPULAR SCIENCE MONTHLY. 



ous. White parts of animals are often attacked with disease, while 

 the other parts remain healthy ; and light-skinned animals are most 

 troubled by flics and parasites. Albinoisra, which is simply a total 

 inaptitude for the jjroduction 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 refract- 

 ing media, for example, may not yet be existing even in a rudiment- 

 ary 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, more- 

 over, 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 dis- 

 tribution of the colored granulations in the integuments. 



On the other hand, certain constitutional defects induce a diminu- 

 tion or absence of coloring-matter, of which I can give no better illus- 

 tration than to cite Darwin's curious observation that white cats gen- 

 erally 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 — cor- 

 respond closely with an abundant production of coloring-matter ; while 

 the negative facts of life — age, constitutional weakness, disease, apa- 

 thy, and degeneration generally — lead to a more or less complete dis- 

 appearance of the same substance. Nevertheless, we notice in some 

 cases the contrary fact, or a deposition of coloring-matter, or an in- 

 crease of its production in connection with some pathological condition 

 of the organism. But these cases, which seem opposed to our theory, 



