may be of direct physiological value to the organism, or may assist in the struggle for existence by deluding other species, or by aiding the individuals of the same species, or they may be intimately connected with courtship.
The color of chlorophyl, which causes the green appearance of vegetation, must be intimately connected with the important changes which take place in this substance in the presence of light. It is well known that under these circumstances carbon dioxide (popularly called "carbonic acid") can be split up, and its carbon made to unite with the elements of water, forming organic substance. Although this process has been much studied, it is still very imperfectly understood. It is clear, however, that the color of chlorophyl, involving the special absorption of certain lightwaves, has some direct bearing upon the changes which occur,
No equally clear instance has been proved to occur in the animal kingdom, except in those few forms which resemble plants in possessing chlorophyl. Dr. Hickson, however, believes that among corals "the most widely distributed colors will eventually be proved to be allied to chlorophyl, . . . and perform a very similar if not precisely identical physiological function." It is much to be desired that this interesting suggestion, which Dr. Hickson supports by many arguments, may be thoroughly tested as soon as possible.
In the very common association of colored substances with the important function of respiration, it is clear that the color is not more than incidental; while the fish with transparent blood shows that color is not indispensable for the due performance of the function. Pigment is, however, of direct importance for vision; it is always present in the eyes of animals, except in the case of albinos, and it is said that even they possess the essential visual pigment associated with the termination of the optic nerve (retinal purple).
The difference between the physiological importance of color in animals and plants is well shown by the fact that a true albino variety (not merely a variegated example) of a green plant could not live for any length of time.
There are, however, certain cases among animals in which it is extremely probable that color is of direct physiological value. It is well known that dark colors readily absorb radiant heat, while light colors do so with difficulty. For this reason black clothes are most trying, and white most comfortable, in the hottest weather. Conversely, a dark surface readily parts with heat by radiation, while a white surface retains heat far more effectually.
A few writers had suggested that these principles may explain
- A Naturalist in North Celebes (Hickson, 1889), pp. 149-151.