the colors of certain animals, but the question was first fully entered upon in Lord Walsingham's presidential address to the Yorkshire Naturalists' Union in 1885. The predominance of dark varieties of insects and white varieties of birds and mammals in northern latitudes is connected with these facts. "Birds and animals living through the winter naturally require to retain in their bodies a sufficient amount of heat to enable them to maintain their existence, with unreduced vitality, against the severities of the climate. Insects, on the contrary, require rapidly to take advantage of transient gleams of sunshine during the short summer season, and may be content to sink into a dormant condition so soon as they have secured the reproduction of their species; only to be revived in some instances by a return of exceptionally favorable conditions."
It would be fatal for the temperature of one of the higher vertebrates to sink a few degrees below the normal, except in the case of certain species, such as the dormouse, etc., which have the power of hibernating in a dormant condition; such animals were once called "warm-blooded," but are now more correctly termed "homothermic," because it is the constancy of the temperature which is so important, and which must be maintained whether the surrounding medium be colder or warmer than themselves. Other animals with an inconstant temperature are now correctly called "poikilothermic" rather than "cold-blooded."
Lord Walsingham's conclusions appear to be supported by the fact that young dark-colored caterpillars, like those of the emperor moth (Saturnia carpini), or tortoise-shell butterfly (Vanessa urticæ), seek the light side of a glass cylinder, and always change their position when the cylinder is turned round. The question needs further investigation, and much might be learned by interposing various screens between such larvae and the light, thus cutting off different sets of light-waves.
The most important support to the hypothesis is found in an experiment made by Lord Walsingham, in which several Lepidoptera of different colors were placed on a surface of snow exposed to bright sunshine; in half an hour the snow beneath the darker insects showed distinct signs of melting, but no effects were seen beneath the others. The differences were further brought out in the course of two hours, when the darkest insect of the lot, a black geometer, the chimney-sweeper (Odezia chærophyllata), "had decidedly won the downward race among them."
It is therefore certain that the absorption of radiant heat is favored by the dark colors of northern insects, and it is in every way probable that they are benefited by the warmth received in
- See Entomological Transactions of the Union for 1885.