COLOR PP.E•ERENCE. Different animals have a marked preference for certain colors of the spec trii Ill, in which they seem to feel better. The little crustacean Daphnia ].refers to swim in orange, yellow. and especially in green light. The starfish slums the red rays. love light, such 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 on respiration. It has been observed that slugs almost always move with reference to certain external agents, as gravity and strong light: they move in straight lines vertically either from or toward the !Hit. It is so also with the Hydra, and starfish, as well as with the larvae of certain starfish.
Counts A xro.u.s 1 N FLU EN CEO BY LI GU T. It is well known that the chameleon and the llori as well as tree-loads. rapidly ehange their color from green to gray or brown. according is they rest among green 11'n VeS or on the t rook or branches of trees. The 'ehromatie function' is that adaptation of the color of the skin of these animals, as well as of squids, crusta ceans, 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-cells. 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, n-hile the covered parts remained dark. To avoid all doubt. the experiments were repeated on skin separated from the body, and photograms of letters and flowers. 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 eve, but in those derived from the skin.
The action of light also causes the varied hues and markings of the chrysalids of butter flies. 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 such pupa are due to exposure to the surroundings. Thus, Poulton found that where the pume 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 more efficient than white in producing •hrysalids 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 is well known. is white. owing to the absence of 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 Pollak. to be blind.
Ily experiments in severing the connection of some of the' spinal nerves with the sympathetic nerves of the same side, Pouchet sueceeded 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 sick of a lisp. while the other side retained its natural hues and their normal variation. according to the colors reflected from surrounding objects.
It is well known that small flounders and vari ous kinds of shrimps will turn pale if placed in a white of salt and the red, green. and brown colors of shrimps and other crustaceans will ehange inn few hours in color io correspond With the green. red. or brown in which they rest. This was carefully observed in the case of the p•aWn by Ilerdman, who also kept a number of specimens of different colors under observation in jars, with various colors of sea weeds and of background, and in very different degrees of light. The results showed that the adult animal can change its coloring very thor oughly, though not in a very short space of time. The change is duo to alterations in the size and arrangement of the pignut-nt-granules of the chroma toph ores.
It will be seen from these facts that the won derful Imes of tropical fish, with their colored hands, the stripes of snakes, or of the tiger, the spots of the leopard, the gay markings of cater pillars and of butterflies, as well as of spiders, are originally clue to changes in light and shade. This will also apply to of the protective markings of insects, and other animals; the initial cause or factor is the varying action of the sun's light, though natural selection may act as a secondary factor, those forms or varieties most protected by their special style of colora tion surviving.