It must be evident from what has already been explained in Chapter III that man, so widely spread and subjected therefore to great differences of climate, consuming very different food in different regions, attacked by different enemies, must always have been undergoing considerable local modification and differentiation. Man, like every other species of living thing, has constantly been tending to differentiate into several species; wherever a body of men has been cut off, in islands or oceans or by deserts or mountains, from the rest of humanity, it must have begun very soon to develop special characteristics, specially adapted to the local conditions. But, on the other hand, man is usually a wandering and enterprising animal, for whom there exist few insurmountable barriers. Men imitate men, fight and conquer them, interbreed, one people with another. Concurrently for thousands of years there have been two sets of forces at work, one tending to separate men into a multitude of local varieties, and another to remix and blend these varieties together before a separate species has been established. These two sets of forces may have fluctuated in this relative effect in the past. Palæolithic man, for instance, may have been more of a wanderer, he may have drifted about over a much greater area, than later Neolithic man; he was less fixed to any sort of home or lair, he was tied by fewer possessions. Being a hunter, he was obliged to follow the migrations of his ordinary quarry. A few bad seasons may have shifted him hundreds of miles. He may therefore have mixed very widely and developed few varieties over the greater part of the world. The appearance of agriculture tended to tie those communities of mankind that took it up to the region in which it was most conveniently carried on, and so to favour differentiation. Mixing or differentiation is not dependent upon a higher or lower stage of civilization; many savage tribes wander now for hundreds of miles; many English villagers in the eighteenth century, on the other hand, had never been more than eight or ten miles from their villages, neither they nor their fathers nor grandfathers before them. Hunting peoples often have enormous range. The Labrador country, for instance, is inhabited by a few thousand Indians,[70] who follow the one great herd of caribou as it wanders yearly north and then south again in pursuit of food. This mere handful of people covers a territory as large as France. Nomad peoples also range very widely. Some Kalmuck tribes are said to travel nearly a thousand miles between summer and winter pasture.