It might still be true that all but the least conspicuous members of the world’s population belong to a single family, or to an extremely small number of separate lines of descent. If we suppose that, as soon as the globe was fitted for living occupants, a single simple organism was spontaneously generated, or, if you please, created, or, in any other way that may be named, introduced upon the earth, the results in accordance with actual facts may be logically deduced agreeably to the various principles of the theory of development. Its descendants would multiply and replenish the earth, unchecked, in all parts suited to their conditions of existence, till all such parts were occupied. No further advance in the population of the world could then take place until some variation had occurred, making possible the occupation of new regions, or of the old regions, under new conditions. But the new species, constituted by some advantageous variation, would be likely to overrun the whole field, to the almost complete suppression of the earlier and more simple form. The distance thus gained in the race of progressive organization it would be likely not only to maintain, but greatly to increase. Its descendants would vary in more than one useful direction, till it might, as we have said, become the parent of all the conspicuous members of the earth’s population. The earliest and simplest form might still have representatives inheriting its likeness by direct descent, but unable to make their way in the world, not from wanting the power to vary, but from finding the world pre-occupied by species too powerful for them to compete with. In this way they would be restrained to their original insignificance. Now exactly the same result would follow, if, instead of being born in what we consider the ordinary course of parentage, these simple forms were ever being spontaneously generated. They would find the world pre-occupied against their advancement in the scale of organization; they would rarely, if ever, be allowed to lead up by successive useful variations to highly organized forms; and if ever, only in periods of time so enormous as to perplex the acutest human understanding.