The view we have now obtained of the specific differences between distinct races of being at separate periods of their existence is precisely what might have been anticipated from the law of gradual development, which holds throughout the organic kingdoms. Between two mature animals, these differences are perfectly obvious and well marked. As we go a step back in their history, the distinction becomes a little more obscure; two worms may resemble each other very closely, though the two winged insects subsequently produced from them may be very unlike. Receding still farther, some of these specific differences may entirely disappear, the organs or parts which should exhibit them being not yet developed. And when we come to the primitive germs, so minute as to be visible only through the microscope, no outward distinction, perhaps, is any longer perceptible, and the radical difference of their internal organization is indicated only by the fact, to be verified by subsequent observation, that the two are invariably developed into perfectly distinct animals, belonging respectively to the same races with their parents. A theorist, whose whole system is based upon the invariable operation of natural agencies, cannot reasonably object to this conclusion. That our statements in the course of this argument may not appear of the same questionable character as those advanced by our author, we will fortify them with a few brief citations from a work of such unquestionable authority as the Lectures of Professor Owen. "No doubt the minute infusoria, which seem to have their development arrested at the first or nearest stage from the primitive cell formation, offer close and striking analogies to the primitive cells out of which the higher animals and all their tissues are developed; but the very [first] step which the infusoria take beyond the primitive cell stage invests them with a specific character as independent and distinct in its nature as that of the highest and most complicated organisms. No mere organic cell, destined for ulterior changes in a living organization, has a mouth armed with teeth, or provided with long tentacula; I will not lay stress on the alimentary canal and appended stomachs, which many still regard as 'sub judice'; but the endowment of distinct organs of generation, for propagating their kind by fertile ova, raises the polygastric infusoria much above the mere organic cell."--pp. 25, 26.