TEMPERATURE VARIETIES. Varieties, races, or sub-species are largely differentiated by variation resulting from being subjected to dif ferent influences, principally in the nature of temperature and comparative moisture. It is a matter of universal observation that widespread species present a great range of variability in size, proportion of parts, color and other char acteristics, so that it is often a matter of doubt whether a variant ought to be classed as a geographical sub-species, or as a distinct species. When it appears that the same kind of variation affects all or nearly all of the animals of a certain class in .a region, coincidently with the character of the local climate, it is fair to as sume that the change from the normal noted is due to local climatic influence. Temperature seems to act most directly in producing the common form of seasonal dimorphism, which affects most of the higher animals that dwell in the temperate zones, and are called upon to en dure the cold of winter as well as the heat of summer. This causes a semi-annual change in mammals and birds from a comparatively thin and short coat of hair or feathers in the warm half of the year to a thicker and longer cover ing more suitable to the cold season. Tn ad dition to this there is always a greater or less change of color, the bright dress of summer, as sumed after the spring molt, falling out in the autumn and being gradually succeeded by a plainer, less conspicuous suit for the winter months. The difference is often very great: and
in the arctic regions amounts to a change from decided colors in summer to pure white in win ter.
The comparative degree of average moisture in the air of two regions will be reflected in the colors and other features of the same species of animals inhabiting them; those of the moist region will have darker colors, and a tendency to larger size and proportionally longer ex ternal parts, as bills, legs, and tails, than the dry land species, which will be more pale in color and compact in form. Dr. J. A. Allen has shown this very conclusively in his investigations upon North American birds. Let one of these districts be severed for a considerable period from the others and the results accredited to isolation (q.v.) would follow; but the species so formed might well be called 'temperature species' in reference to their origin. Geographical races may therefore be regarded as incipient species.
That changes from beat to cold, wetness to dryness, or the reverse, in the climates of re gions of the world in the course of its history since animal life appeared upon it, have been efficient factors in species-making, may well be believed.
See EVOLUTION ; MELANISM ; ISOLATION; and consult authorities there mentioned.