The relation of life to the earth's area is a fundamental question of bio-geography. The amount of that area available for terrestrial life, the proportion of land and water, the reduction or enlargement of the available surface by the operation of great cosmic forces, all enter into this problem, which changes from one geologic period to another. The present limited plant life of the Arctic regions is the impoverished successor of a vegetation abundant enough at the eighty-third parallel to produce coal. That was in the Genial Period, when the northern hemisphere with its broad land-masses presented a far larger area for the support of life than to-day. Then the Glacial Period spread an ice-sheet from the North Pole to approximately the fiftieth parallel, forced back life to the lower latitudes, and confined the bio-sphere to the smaller land-masses of the southern hemisphere and a girdle north of the equator. The sum total of life on the globe was greatly reduced at the height of glaciation, and since the retreat of the ice has probably never regained the abundance of the Middle Tertiary; so that our period is probably one of relative impoverishment and faulty adjustment both of life to life and of life to physical environment.[292] The continent of North America contained a small vital area during the Later Cretaceous Period, when a notable encroachment of the sea submerged the Atlantic coastal plain, large sections of the Pacific coast, the Great Plains, Texas and the adjacent Gulf plain up the Mississippi Valley to the mouth of the Ohio.[293]
The task of estimating the area supporting terrestrial life which the earth presented at any given time is an important one, not only because the amount of life depends upon this area, but because every increase of available area tends to multiply conditions favorable to variation. Darwin shows that largeness of area, more than anything else, affords the best conditions for rapid and improved variation through natural selection; because a large area supports a larger number of individuals in whom chance variations, advantageous in the struggle for existence, appear oftener than in a small group. This position is maintained also by the most recent evolutionists.[294]
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.
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.
Area an index of social and political development.
The successive stages of social development—savage, pastoral nomadic, agricultural, and industrial—represent increasing density of population, increasing numerical strength of the social group, and finally increasing geographical area, resulting in a vastly enlarged social group or state. Increase in the population of a given land is accompanied by a decrease in the share which each individual can claim as his own. This progressive readjustment to a smaller proportion of land brings in its train the evolution of all economic and social processes, reacting again favorably on density of population and resulting eventually in the greatly increased social group and enlarged territory of the modern civilized state. Hence we may lay down the rule that change in areal relations, both of the individual to his decreasing quota of land, and of the state to its increasing quota of the earth's surface is an important index of social and political evolution. Therefore the rise and decline not only of peoples but of whole civilizations have depended upon their relations to area. Therefore problems of area, such as the expansion of a small territory, the economic and political mastery of a large one, dominate all history.