"It is necessary now to discuss plainly what is meant by a phrase, used often very carelessly, "The Races of Mankind". 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 series has been established."