The warrant for seeking a further classification is twofold. It is a principle in science that classification should rest on the most basal characteristics. To determine what these are may not always be easy, but it is at least evident that a classification framed on the ultimate nature of organisms must be more distinctive than one based on external characters. Before the principles of classification were understood, organisms were invariably arranged according to some merely external resemblance. Thus plants were classed according to size as Herbs, Shrubs, and Trees; and animals according to their appearance as Birds, Beasts, and Fishes. The Bat upon this principle was a bird, the Whale a fish; and so thoroughly artificial were these early systems that animals were often tabulated among the plants, and plants among the animals. "In early attempts," says Herbert Spencer, "to arrange organic beings in some systematic matter, we see at first a guidance by conspicuous and simple characters, and a tendency toward arrangement in linear order. In successively later attempts, we see more regard paid to combinations of characters which are essential but often inconspicuous; and a gradual abandonment of a linear arrangement for an arrangement in divergent groups and re-divergent sub-groups."[96] Almost all the natural sciences have already passed through these stages; and one or two which rested entirely on external characters have all but ceased to exist--Conchology, for example, which has yielded its place to Malacology. Following in the wake of the other sciences, the classifications of Theology may have to be remodeled in the same way. The popular classification, whatever its merits from a practical point of view, is essentially a classification based on Morphology. The whole tendency of science now is to include along with morphological considerations the profounder generalizations of Physiology and Embryology. And the contribution of the latter science especially has been found so important that biology henceforth must look for its classification largely to Embryological characters. But apart from the demand of modern scientific culture it is palpably foreign to Christianity, not merely as a Philosophy but as a Biology, to classify men only in terms of the former. And it is somewhat remarkable that the writers of both the Old and New Testaments seem to have recognized the deeper basis. The favorite classification of the Old Testament was into "the nations which knew God" and "the nations which knew not God"--a distinction which we have formerly seen to be, at bottom, biological. In the New Testament again the ethical characters are more prominent, but the cardinal distinctions based on regeneration, if not always actually referred to, are throughout kept in view, both in the sayings of Christ and in the Epistles.