On purely geographical grounds, also, a large area stimulates differentiation by presenting a greater diversity of natural conditions, each of which tends to produce its appropriate species or variety.[295] Consider the different environments found in a vast and varied continent like Eurasia, which extends from the equator far beyond the Arctic Circle, as compared with a small land-mass like Australia, relatively monotonous in its geographic conditions; and observe how much farther evolution has progressed in the one than in the other, in point of animal forms, races and civilization. If we hold with Moritz Wagner and others that isolation in naturally defined regions, alternating with periods of migration, offers the necessary condition for the rapid evolution of type forms, and thus go farther than Darwin, who regards isolation merely as a fortunate contributory circumstance, we find that for the evolution of mankind it is large areas like Eurasia which afford the greatest number and variety of these naturally segregated habitats, and at the same time the best opportunity for vast historical movements. [Sidenote: The struggle for space.] Evolution needs room but finds the earth's surface limited. Everywhere old and new forms of life live side by side in deadly competition; but the later improved variety multiplies and spreads at the cost of less favored types. The struggle for existence means a struggle for space.[296] This is true of man and the lower animals. A superior people, invading the territory of its weaker savage neighbors, robs them of their land, forces them back into corners too small for their support, and continues to encroach even upon this meager possession, till the weaker finally loses the last remnant of its domain, is literally crowded off the earth, becomes extinct as the Tasmanians and so many Indian tribes have done.[297] The superiority of such expansionists consists primarily in their greater ability to appropriate, thoroughly utilize and populate a territory. Hence this is the faculty by which they hasten the extinction of the weaker; and since this superiority is peculiar to the higher stages of civilization, the higher stages inevitably supplant the lower. [Sidenote: Area an index of social and political development.]