DISTRIBUTION OF LIVING MAM MALS. Mammals live in practically all parts of the earth, both on land and in the sea, excepting the icy deserts of interior Greenland and about the South Pole. The diversity of climate and other physical conditions within this vast area, during both past and present time, has furnished an endlessly varied environ ment and the characters of the multitude of suc cessively extinct mammals in the fossil beds of the geologic ages show that the group of mam malia has always had a most plastic organism. The mammalian types now inhabiting the earth by their wonderful variety present living evi dence of this fact. They range from the tiny shrew weighing less than an ounce to the ele phant and the whale, weighing tons; from the clumsy mole to the winged bat, excelling the birds in some of its powers of flight; and from the stupid sloth to man. As we see them mam mals appear to fit so perfectly into their en vironment that at first glance the various species might be considered to have always ex isted in their present haunts. The occurrence of closely related living species in widely separated areas and the evidence of the fossil records in dicate strongly that the ancestors of much the greater proportion of living mammals probably had their origin far from their present homes. The distribution of existing mammals is merely the latest chapter of a wonderful story leading back through the geologic ages and involving almost incredible changes in the earth's surface and the relationships between land and sea. The fossil beds prove that in practically all regions one fauna has succeeded another, often of the most diverse character, in a marvelous procession.
So far as distribution alone is concerned the mammals may be divided roughly into two main categories, each rather definitely limited. First, those which are characterized by having the four limbs more or less completely developed for life on the land. Second, those which have all of their limbs greatly modified, and sometimes with the posterior pair lacking, in order to fit them for life in the sea. Land mammals, living among infinitely diversified conditions, are correspondingly much varied in size, form, color, specialization of organs and habits. Sea mammals living under far more uniform conditions present much fewer and more uniform types which are included within the three great groups, the whales, the pinnipeds or seals, and the sirenia or manatees and their relatives. The seals and larger whales arc most
characteristic of the colder northern and south ern seas, while the smaller whales or dolphins and the manatees are most characteristic of the warmer seas and some of their tributary rivers. The present distribution of land mammals has resulted from the interaction of numerous causes many of which are little understood or are unknown. Temperature is known to be the most potent single influence. A long continued change of 10° F. in the average summer temperature of any region would have a marked influence on its mammal life; many species would disappear and be replaced by others. We have reason to believe that such changes in temperature have been the primary causes of many of the great changes in mam malian faunas in various parts of the earth which have been revealed by studies of the fos sil beds. The distribution of species is strongly affected also by the prevalent degree of atmos pheric humidity, as indicated by the contrasting faunas of hot desert areas and of the mighty forests of the humid tropics. Oceans, great rivers, high mountain ranges and elevated table lands form barriers which also affect the dis tribution of mammals. The Sahara Desert is believed to have existed since a remote period and forms an effectual barrier against the spread of mammals living adjacent to its borders. In the same way great forested areas in other parts of Africa and elsewhere have their peculiar species and bar the way to species living on the open plains. Elevated mountain ranges, especially those extending in a northerly and southerly course, and high interior table lands directly influence climate conditions and the animal life on them. For instance the great mountain system of western North America carries southward to within the tropics repre sentatives of the characteristic northern mice of the genus Microtus. The broad interior table land of Mexico carries southward from the southwestern border of the United States to the valley of Mexico the pocket mice (Perogna thus), the kangaroo rats (Dipodomys) and others, while along both lowland coasts of that country tropical species of mammals such as peccaries (Pecan) and tiger cats (Felis) range northward to our border.