The Dawn of Reason; or, Mental Traits in the Lower Animals

by Jr. James Weir

When speaking of light, if not otherwise specified, I mean diffused daylight which carries no heat rays. I believe that heat is a prominent factor in the production of color; the discussion of this point, however, does not properly belong to the subject under consideration.

Some experiments on newts, made by myself several years ago, show that the absence of light does not influence pigmentation,—that is, through several generations. My animals were kept under observation from the extrusion of the eggs until full maturity had been reached, and great care was taken to make experiments as accurate and as conclusive as possible.

Those reared in total darkness or in a red light were always dark-colored; those reared in a yellow light[96] were almost but not quite as dark; while those reared in white ironstone crocks and in diffused daylight were very much lighter, being pearl-gray in color. This apparent (for the microscope showed that it was only apparent) absence of color in the last-mentioned specimens was due to tinctumutation.

In most viviparous animals the embryo is developed in almost or absolutely total darkness, yet when it is born it has bright colors. Kerbert has found in the cutis of the embryonic chick, about the fifteenth day, certain pigment-cells. These cells have entirely disappeared by the twenty-third day. It is probable that little, if any, light can reach the chick through the shell and membranes, yet pigment-cells develop and disappear again.[97]

A butterfly emerges from the cocoon arrayed in all the colors of the rainbow; yet it was developed, while in the pupa state, in total darkness. It is not necessary to mention further instances; we readily see that pigmentation in animals is not necessarily dependent on light. Neither is tinctumutation the result of the direct influence of light on the chromatophores. Light, however, if not the direct, is the indirect cause of this phenomenon. Lister, in 1858, showed that animals with imperfect eyesight were not good tinctumutants, notwithstanding the fact that they had the chromatophoric function. He showed, by his experiments on frogs, that the activity of the chromatophores depended entirely on the healthy condition of the eyes,—that is, so far as the phenomenon of tinctumutation was concerned. So long as the eyes remained intact and connected with the brain by the optic nerve, the light reflected from the surrounding objects exerted a powerful influence on the chromatophores. As soon as the optic nerve was severed, the chromatophores ceased to respond to the influence of light and color, no matter how bright and varied they were. The deductions drawn from these experiments are not to be controverted or denied. The chromatophores are influenced by light reflected from objects and transmitted via the optic nerve to the brain; from this organ the impression or irritation goes to the nerve governing the contractile fibres of these pigment-holding glands.[98]

Pouchet followed Lister, and confirmed his conclusion by experiments on fishes and crabs. He remarked that the plaice—a fish with a white under-surface and a party-colored back—had the chromatophoric function highly developed. Among a number of specimens which appeared pale on the white, sandy bottom, he met "one single dark-colored fish, in which, of course, the chromatophores must have been in a state of relaxation; and this specimen was as distinct from its companions as from the bottom of the aquarium. Closer investigation proved that the creature was totally blind,[99] and thus incapable of assuming the color of the objects around it, the eyes being unable to act as a medium of communication between them and the chromatophores of the skin."[100] Thus far Pouchet had only confirmed Lister's observations, although it is highly probable that he was unaware of Lister's experiments. But he went a step further. There are two ways in which cerebral impressions may be transmitted from the brain to the skin: one, by way of the spinal cord and the pairs of nerves arising from it and known as spinal nerves; the other, by two nerves running close to the vertebral column—the sympathetic nerves.

Pouchet cut the spinal cord close to the brain, yet the chromatophores still responded to light impression, showing that they did not receive the message through the cord and spinal nerves. He then divided the sympathetic nerves, and the chromatophores lost at once the power of contraction; he thus demonstrated that the sympathetic nerves were the transmitters of the optical message, and not the cord.

This discovery of Pouchet is, psychologically, of great importance, though he failed to recognize it as such. He was satisfied with its anatomical and physiological significance.

When we remember that the actions of the sympathetic nerves are almost, if not entirely, reflex in character, we at once see the psychological importance of this discovery. This fact makes the phenomenon of tinctumutation an involuntary act on the part of the animal possessing the chromatic function, and thus keeps inviolate the fundamental laws of evolution, which, were the facts otherwise, would be broken.[101]

By a series of experiments on frogs I have confirmed the conclusion of Pouchet in toto, and have even solved, so I believe and unhesitatingly assert, the puzzling problem of the physiological modus operandi of the wonderful phenomenon of tinctumutation.