The simple fact of a population becoming more numerous, necessitates certain changes--from hunting to pasturage, for example, from pastoral life to agricultural and fixed habitation--and these would affect the habits, modes of thought, and, to some extent, personal appearance. The modification of climate by clearing, draining, and cultivation, and the removal of a people from one climate to another, would effect still other changes. But the intermixture of races by war and immigration has, perhaps, done more than any other cause to produce the great physical diversities which we now find in the higher races. Having traced the stream of warlike immigration from Eastern Asia westward, and thence to Central Europe, and still westward and southward to the shores of the Atlantic, and even across the Mediterranean into Africa, overwhelming the Roman Empire of the West in its course,--observe this tide of human movement, as wave followed wave for centuries, rolling peoples against and over one another, confounding them together, and leaving them upon the same soil, or in close proximity to each other; and, even admitting that they were simple and primitive to begin with, we shall not wonder at the diversified aspect of the people of Europe and their descendants in America. But this is only one series of movements from which has resulted the intermixture of races; there are others, and some, no doubt, beyond the farthest reach of history. The process of intermixture is still going on, especially in the Western World, though by methods usually more peaceful than formerly. The result multiplies itself, and the leading races of mankind are becoming constantly more composite. The contact and intermixture of races have had a moral result, which, in its turn, acts upon the physical. Mental development has been one of the results of war and immigration; one people learning from another, and striking out new modes of thought from the sheer necessity of new circumstances; and this mental development changing the physiognomic expression and general bearing of the man. This result has been increasing in geometrical progression since history, printing, and the facilities of intercommunication have made the culture of one people contagious to other peoples, and the attainments of one generation available to all the generations that follow. Thus does every movement among the nations conspire to change the simple types into those which are more complex. The ethnological unity may be less apparent; and before we clearly perceive it, we may have to rise into the consideration of social and political relations, not divorcing these from physiology, without which no question relative to man can be rightly judged. And it may be that after greater development in this direction, the unity of races may become more distinctly pronounced and more readily recognized.