GEOGRAPHIC DISTRIBUTION 321 



in Australia, the cotton boll weevil in southern United States, and the 

 English sparrow in America. 



World-wide Scheme of Distribution. — Having so far examined some 

 of the peculiarities of distribution, and the biological or geological 

 processes needed to explain them, we may now attempt to see how these 

 interlocking phenomena affect distribution on a large scale. One must 

 usually limit such a study to a single major group of animals because 

 of the different timing of evolutionary events in relation to changes in 

 the earth. Zoogeographers have proposed different groups for this pur- 

 pose. Mammals, snails, earthworms, birds, reptiles, insects, all have 

 been urged as suitable. We shall use mammals, primarily because the 

 different kinds are better known among nonbiologists, but partly because 

 they are large, and the world has been explored enough to discover the 

 location of most of them. They have one further advantage: their 

 evolution has been rapid and recent, so that the effects of changes of 

 the earth will be more readily discovered than in groups whose evolution 

 has been slow and protracted. 



The bulk of the land area of the earth is in the northern hemisphere. 

 With the connection which must have existed across Bering Strait, this 

 land was formerly a continuous body. From this area there project 

 southward three great continental masses, South America, Africa, and 

 Australia. The last is believed to have been connected with Asia across 

 the Malay Archipelago prior to Jurassic time. South America, though 

 now connected with North America, is held to have been separated 

 from it in early Tertiary time. This is indicated by similarity of the 

 marine animals on the east and w^est coasts of Central America, as well 

 as by geological evidences. 



Origin of Mammals. — Primitive mammals are believed to have arisen 

 first in the northern continents. This conclusion flows partly from 

 theory, since the great variations of environmental conditions character- 

 istic of huge land masses should have been able to act selectively on 

 almost any type of evolutionary change w^hich happened to occur in 

 living things. The northern continental mass as the place of mammalian 

 origin is supported, moreover, by the fact that the most primitive fossils 

 of the group have been found there, though it must also be said that 

 more explorations have been made in that area. 



These primitive mammals, resembling our monotremes and mar- 

 supials more than true mammals, must have spread in all directions. 

 To the north barriers were soon reached, but to the south the three 

 great prongs of land provided ample room; and they had a geological 

 age or two in which to enter these. 



Then the higher (true) mammals began to arise, also in the northern 

 land mass. They proved to be superior to their predecessors, that is, 



