A brief glance may still more closely illustrate this analogy between the geological series and the organic systems. Plants and animals seem to have appeared nearly at the same time, and at first in the form of the very lowest organisms. The earliest plants found by geology belong also to the lowest stage of the vegetable kingdom; they are the algae. They are followed again by higher cryptogamous plants, especially ferns and club-mosses. Only at a later period flowering plants appear, among them being first the plants with naked seeds standing lower in the systems, as the cycad-trees and pine-forests; later, those with enclosed seeds, among them being again first the monocotyledons, last the dicotyledons,--all of them precisely corresponding to the botanical system. The same thing is true in the animal kingdom. If the eozoon Canadense, found in the laurentian slate of the Cambrian formation in North America, is really an organism and not an inorganic form, the earliest vestiges of animal life we can find are the rhizopodes or foraminifera; and these organisms belong to the lowest stage of life--to that stage which forms a kind of undeveloped intermediate member between the vegetable and animal kingdom, Haeckel's kingdom of the protista. The next oldest animal organisms found in the Cambrian formation are the zooephytes, and immediately above them the mollusca and the crustacea. In the following Silurian period we find corals, radiata, worms, mollusca, and crustacea, in {66} great number, also all the main-types of the invertebrates; and in the highest Silurian strata there are also to be found representatives of the lowest class of vertebrates, of fish, but still of very low organization and little differentiated. That the five main-types of the invertebrates seem to have appeared quite contemporaneously, yet that the zooephytes really appeared first, does not contradict the before-mentioned law of a progress in the appearance of the organisms from the lower to the higher. For in the zooelogical system also these main-types of the invertebrates do not stand one above the other, but by the side of each other: at most, the radiata, the worms, the mollusca, and the articulata, take their places above the zooephytes. Only within the main-types, in the classes, orders, etc., do differences in rank take effect; and even here, not without exception. What difference in rank, for instance, is there between an oyster and a cuttle-fish? between a cochineal and a bee or ant? and yet the first two belong to one and the same type--the type of mollusca; and the last three to one and the same class--the class of insects. The vertebrates rank decidedly above the invertebrates; and in a manner wholly corresponding to this, the vertebrates also appear after the invertebrates. Just as decidedly as to their rank, the main classes of the vertebrates do not stand beside, but above one another: above the fish stand the amphibia, above them the