EVOLUTION. 324 EVOLUTION. Effects of Changes of Light. Our light comes from the sun. Without sunlight there would be no life. Were it not for the stimulus of the sun's rays, animals would be eyeless; and it is the absence of light which bleaches animals liv- ing in darkness, preventing the formation and development of the pigment in the skin. On the other hand, the colors of animals, the beautiful and varied tints of butterflies and bird-, of tropi- cal shells as well as the gorgeous hues of flower-. are all due to the stimulating effects of light. Light is necessary for the development of chlorophyll, and therefore is an indispensable agent in the life of all green plants, and espe- cially for tree-life, and wood-formation. Sachs observes that "the forms and modes of life pre- sented by plants . . . must have been to a large extent induced by the continued action of. gravitation and light." And this applies as a general statement to the simple animals, and in a sense to all animal life. The influence of changes in the environment, and especially of light, on organism, may be im- mediate and direct, or it may be indirect. The French botanist Bonnier has shown by experi- ments that a life in feeble light produce- a change in the structure of plants, and that if placed in markedly different surroundings they either perish or become rapidly modified to meet the changed conditions. He has found that Arctic plants differ noticeably from the same species growing in Alpine regions in the greater thick- ness and simpler structure of the leaves, and has attributed this to the feeble light of the Arctic region and to the greater degree of moisture. By means of feeble electric lighting and a moist, cool temperature he had in his laboratory in Paris produced these differences, i. e. converted Alpine into Arctic plants. Phototaxis. This means the influence of light in directing the movements of freely moving cells or unicellular plants, as certain bacteria. 'Helio- tropism' means a turning to the light, as seen in the case of infusoria and many higher ani- mal- which will in an aquarium press to the light and collect on the side of the vessel next 1o the light. The influence of light on night- flying moths and other insects i- well known. See Tbopism. Color Preference. Different animals have a marked preference for certain colors of the spec- trum, in which they seem to fee] better. The little crustaaean Daphnia prefers to swim in orange, yellow, and especially in green light, starfish shuns the red rays. Animals which love light, -uch as bees, prefer blue or green, while the light-shunning forms, such as ants, have less antipathy for red than for other colors. In these eases light doubtless acts as an excitant en respiration. It has been observed thai slugs o i always move with reference to certain in.'il agents, as gravity and strong light; they move in straight lines vertically either from toward the light, it is so also with the Hydra, and starfish, as well as with the larva: 1 1 a in starfish. oi Animals [nfxtjenced nv Light. It i i hat i he chameleon and t he Flori- dan Vnolis lizi -11 as tree-toads, rapidly from green to gray or brown, ■ i among green lea res or on the trunk nr branches of tree Hie 'chromatic function' i- that adaptation of tl olor of the skin of these animals, as well as of squids, crusta> eeans. etc., to that of their surroundings. This is due to the contraction or expansion of the pig- ment-cells (ehromatophores) in the skin. The pigment differs in color in different individuals and species, and in different parts of the body, being yellow, brown, black, sometimes even red or green. On their distribution and their alter- nate expansion and contraction under the influ- ence of the nervous system depends the pattern which the frog's skin displays at any given mo- ment. It is claimed by Biedemann that the color- cells change their shape as the result of the direct action of light and temperature. It appears that the slightest change of temperature affects the mutual disposition of the pigment-cell-, and consequently the color, of the frog. Keeping the animal in the hand is enough to provoke a con- traction of the black cells. That the light acts as a direct stimulus has been proved by Steinach, who glued strips of black paper to the skin of frogs which were kept in the dark; when they were exposed to the light only the uncovered parts of their skin be- came paler, while the covered part- remained dark. To avoid all doubt, the experiments were repeated on skin separated from the body, and photograrns of letters and flower-, cut out of black paper and glued to the skin, were repro- duced upon it. Besides, blind tree-frogs do not become darker, as fishes do; and Biedemann has proved that the chief agency of their changes of color is not in the sensations derived from the eye. but in those derived from the skin. The action of light also causes the varied hues and markings of the ehrysalids of buttei Hies. During the semi-pupal state, before the chrysalis is fully formed, the surface is, so to speak, photographically sensitive to the color of the surroundings, and the gay hues of -uch pupae are due to exposure to the surroundings. Thus, Poulton found that where the pupa' transformed in boxes, lined with black paper, they became dark, while white light produced pale ones, many of the last being brilliantly golden; this sug- gested gilt surroundings, which were far nunc efficient than white in producing ehrysalids of a distinctly golden color, and even of a deeper hue than often occurs in nature. The under side of flounders and other flatfish, as i- well known, is white, owing to the absence oi pigment, while the upper side of the body is dark, or variously spotted, or eyed. When the under side is dark, the fish will be found, accord- ing to Pouchet. to be blind. By experiments in severing the connection of s if the spinal nerves with the sympathetic nerves of the same side, Pouehel succeeded in limiting the chromatic function to those spots where the nerves remained in connection with the sympathetic; and he was thus able to pro- duce at pleasure a zebra-like marking on one side oi a fish, while the other side retained its natural hue- and their normal variation, according to the colors reflected from surrounding objects. It is well known that small flounders and vari- ous kind- of shrimps will turn pale if placed in a white dish of salt water, and the red, green and blown colors of -blimp- and other crustaceans will change in a few hours in color to correspond with the green, red, or brown seaweeds in which they rest. This was carefully observed in the
