abundant means of critically testing the theory of evolution. If the theory be correct it should supply the key to understanding many phenomena of distribution which would be otherwise unintelligible. If it affords this key the presumption in favor of its truth would be very great.

Mr. Darwin seems to have been the first man who adequately grasped this general fact. In his two chapters on "Geographical Distribution" in The Origin of Species he showed how the salient features of distribution receive a natural and rational explanation according to the theory of evolution. It is a familiar fact that the great continental regions of the earth possess their own peculiar types of fauna and flora. Students of distribution divide the earth's surface into five or six principal regions, each of which is subdivided into several minor areas. The main regions usually adopted are (1) the Palæarctic, which includes Europe and Asia north of the Himalaya Mountains, and Africa north of the Tropic of Cancer; (2) the Nearctic, or North America with the exception of Central America and a part of Mexico; (3) the Neotropical, or South America plus the aforesaid areas of Central America and Mexico; (4) the Ethiopian which includes the part of Africa south of the Tropic of Cancer; (5) the Oriental, consisting of India, Indo-China, and some of the islands to the south and east; and (6) the Australian, which includes Australia and some of the outlying islands mainly to the north and east of the continent.

On account of the general similarity of their faunas and floras the Palearctic and Nearctic regions are sometimes united into a single region called the Holarctic. In both Europe and North America we find related but generally distinct species of bears, wolves, wild cats, foxes, weasels, squirrels, hares, rabbits, deer, moles, and otters. In the forests of both continents we meet with oaks, sycamores, willows, maples, and alders; and among the smaller flowering plants there is a large number of allied species. A few families represented in North America—namely, those including the opossums, humming birds, and prairie dogs are not found in Europe. Nevertheless, students of dis

tribution are abundantly justified in regarding the Palearctic and the Nearctic regions as more closely related than any two of the other great geographical areas.

When we compare the regions lying in the southern hemisphere we find a very different situation. In the continent of South America there is a marvelous wealth of unique forms of plant and animal life. Among the mammals, to mention only a few outstanding examples, there are the llamas and alpacas, sloths, anteaters, armadillos, agoutis, peccaries, bloodsucking bats, and prehensile-tailed monkeys. In the bird fauna, which includes an unusually large proportion of brilliant and conspicuous forms, there is a very large number of peculiar species, "a series of types," according to Wallace, "more varied and more distinct from those of the rest of the world than any other continent can boast of." The reptiles, amphibians, fresh-water fishes, and insects consist also of species usually found nowhere else in the world.

If now we pass across the Atlantic to the Ethiopian region we shall be introduced to a distinct new world of life. The larger mammals are very different from those of South America. Instead of the prehensile-tailed monkeys, which belong to a unique subdivision of the Primates, we find, on the one hand, the baboons and the large anthropoid apes such as the gorilla and the chimpanzee, and, on the other, the group of lemurs which occurs nowhere in the Western Hemisphere. While bears, deer, and wild oxen are conspicuous by their absence, there is an unusually large proportion of large quadrupeds, which include the elephant, rhinoceros, hippopotamus, giraffe, lion, leopard, zebra, okapi, wild ass, and numerous species of antelopes. The bird fauna is no less rich and unique, and there are large numbers of peculiar species of reptiles, fishes, insects, and land shells.

Taking a voyage to Australia, we encounter a fauna and flora still more unique. The mammals, with the significant exception of some bats and a few species introduced by man, consist exclusively of marsupials and monotremes. Mammalian life in Australia has become diversified in ways which more or less