+273. Slavery and polygamy.+ Such instances show us the existence in human nature of a tendency of stronger groups to exploit weaker ones in the struggle for existence; in other words, slavery or forced labor is one way in which, in elementary civilization, the survival of the fittest group is brought about. The slavery of individuals has not the same definite result on the competition of life. "We find polygamy and slavery continually at work dissolving the cohesion of old political institutions in the old civilized races of Asia and Africa. In an uncivilized society, like that of Zululand, they prevent such cohesion ever taking place. They help to keep the Kaffir tribes in perpetual unrest and barbarism, by destroying the germs of civilization and preventing its growth."[647] That the two have this effect in common may very probably be true, but in many respects they are antagonistic to each other. Slavery meets the necessity for many laborers which may otherwise be a cause for polygamy. Wherever slavery exists it affords striking illustrations of the tendency of the mores towards consistency with each other, and that means, of course, their tendency to cluster around some one or two leading ones. Africa now furnishes the leading proofs of this. The negro society is one in which physical force is the chief deciding element. The negroes have enslaved each other for thousands of years. Very few of them have ever become slaves to whites without having been previously slaves to other negroes. In 1875 it was reckoned that twenty thousand persons, chiefly women and children whose male relatives had generally been killed, were taken into slavery from around Lake Nyassa. The difficulties and expense of the slave trade in that region became so great that it could not be carried on except by alliance with one tribe which defeated and enslaved another and sold the survivors. The Arabs opened paths for ivory hunting. The slave dealers used these means of communication. They established garrisons in order to exploit the territory, and ended by depopulating it.[648] Junker argues earnestly against the impression which has been established in Europe that Arabs are chiefly to blame for slavery. "There are places in Africa where three men cannot be sent on a journey together for fear two of them may combine and sell the third."[649]