The accumulation of climatologic influences, and the angle of the line of migration.—Other conditions being equal, why do two tribes under the same degree of latitude differ? e.g. Why are not all tribes under the equator like the Negro of the Niger, and vice versâ?
Without venturing upon the enumeration of all the elements of this difference, I will indicate one, assuming only that the climatological influences of a certain degree of latitude have some effect, and that some effect must be the result of the force in question. I call it the accumulation of climatologic influences.
Let a certain locality under a given degree of latitude (say the west-coast of Africa, under the equator) be peopled by a line of population migrating from Denmark, under one supposition, and from Bombay, under another, the line of migration being, for convenience sake, supposed to be a straight one.
From Denmark, such a line, at its junction with the point in question (say the mouth of the Gaboon River), would form with the equinoctial line, and with each intermediate degree of latitude, a right one.
From Denmark, the angle would be, a very acute one.
Now, just as the angle formed by the line of latitude and the line of migration is acute, the approach made by a moving population towards any particular point under that line (of latitude) is gradual, and in proportion as such an approach is gradual, the number of generations over which a condition of climate, like that of the final point, has been acting is increased; and in this way its influences become accumulated.
Thus, assuming Bombay to be the original cradle of our species—
The Gaboon Negro is the descendant of ancestors who, before they reached their present abode, had moved in a line lying almost wholly within the tropics; whereas—
The American of Quito, is the descendant of ancestors who passed through the tropics by the shortest cut (i. e. at nearly a right angle with the equator), themselves descended from progenitors upon whom the influences of the several North-American, Arctic, and Siberian climates had been at work.