ORIGIN OF COLOR IN ANIMALS. 95 



nae, dependent upon the reciprocal action of parallel waves of light of 

 different velocities, and capable in their different combinations of pro- 

 ducing all the colors of the rainbow, or the absence of color, furnish 

 that dazzling chromatic gamut which Nature employs to paint the 

 humming-bird and the butterfly, those two jewels of the organic 

 world. Another class of phenomena has been called cerulescence by 

 M. G. Pouchet. It is a property which he regards as analogous to fluo- 

 rescence, and as due, in the majority of cases, to stick-shaped bodies 

 inclosed in special cells called iridocytes. The blue reflections pre- 

 sented by the scales of most fishes, the blue color of the caruncles of 

 many birds, and the naked parts of some monkeys, the azure tint of 

 the veins of individuals of the white race, the blue of the iris of some 

 persons' eyes, are examples of cerulescence. These phenomena, how- 

 ever, differ but little from those which give to water having drops of 

 milk suspended in it a bluish color by reflection, or from those which 

 make smoke appear blue when seen upon a black ground. It seems 

 to me that it may be going too far to compare such phenomena with 

 fluorescence. 



These causes of coloration may be superposed and combined in a 

 thousand ways. When birds are under the influence of physiological 

 excitations like those of rage or love, the flow of blood contributes to 

 enliven the color of the bare parts to the point of greatly modifying it. 

 The bright, metallic tints of the peacock and the humming-bird are 

 due to phenomena of interference and to the presence of a dark pig- 

 ment combined ; the green tint of the lizard to the association of a 

 yellow pigment and blue-reflecting iridocytes. The Annelids and the 

 Nemertes, of the invertebrates, exhibit the combined effects of three 

 causes of coloration : iridization, produced by the thin cuticle ; the 

 rich pigmentation of the dermis, and frequently, also, in case the in- 

 teguments are transparent, the variable coloration of the sanguineous 

 fluid and of the internal organs. 



The intensity of coloration is generally proportioned to the vital 

 activity. As life begins to decline, the pigment retires from the for- 

 mations of the epidermis ; and the hairs on regions which have passed 

 maturity often exhibit a lighter coloring than on the neighboring 

 regions. According to Pruner-Bey, the intensity of the color of the 

 negro is an indication of his health ; old negroes grow pale as they 

 age. It is well known that pain and depressing moral trials, which 

 are negative facts in life, provoke the retraction of the pigment. On 

 the contrary, everything that tends to accentuate life occasions an 

 enlivening of the intensity of colors, a fact of which Darwin gives 

 many examples in his " Variation of Animals and Plants." Coloration 

 is strongest in adult animals. Breeders prefer animals rich in pig- 

 ment-matter, because they will best resist disease, and most easily 

 accommodate themselves to special systems of feeding. The ancients 

 regarded animals having white hair on a black skin as the most vigor- 



