On the contrary, they hold that the present conformation and composition of the earth's crust, the distribution of land and water, and the infinitely diversified forms of animals and plants which constitute its present population, are merely the final terms in an immense series of changes which have been brought about, in the course of immeasurable time, by the operation of causes more or less similar to those which are at work at the present day." The idea expressed by the term development involves the same principle, but it is usually restricted to the evolution of organic beings. We will first consider the doctrine as applied to the development of the various forms of life, and then in its broader aspects as a theory of universal evolution.-It has been proved by geology that the earth and its life, instead of being called suddenly into existence a few thousand years ago, have existed for millions of years; and as the mountains and continents are known to have attained their present form by the action of natural agencies, it is thought probable that other objects of nature have been produced in a similar way.

The earth has teemed with living beings through incalculable periods of time, and fossil remains of them are found distributed through the rocky layers that have been successively formed, until they are several miles in thickness. But not all kinds of animals and plants existed from the beginning, leaving their mingled remains in the lowest strata; the lowest types of life, vegetable and animal, appeared first. The successive phases of life are so . definite that they have been held as marking off the earth's history into a series of ' ages. The invertebrates (radiates, mollusks, and articulates) are found in the Silurian or oldest stratified rocks; and from the predominance of the mollusks the period has been called the age of mollusks. Fishes, which are higher in the scale, begin to appear in the Silurian, but become so abundant in the later Devonian period that it is called the age of fishes. Amphibious animals, as an advance on the fishes, appear in the carboniferous age, which again is followed by the age of reptiles. To this succeeds the age of mammals, and lastly comes the age of man, the series, which began with the lowest forms of life, terminating with the highest.

That the order has been progressive, and that its lower terms have been more general in character, while the later terms have been more specialized and perfect, is admitted by all naturalists. Prof. Owen says: In regard to animal life and its assigned work on this planet, there has plainly been an ascent and a progress in the main;" and he has never omitted a proper opportunity for impressing the results of observation showing the more generalized structure of extinct as compared with the more specialized forms of recent animals." Prof. Agassiz holds that the more ancient animals resemble the embryonic forms of existing species;" that is, are lower in the scale of development than the later forms. Mr. Wallace remarks:As we go back into past time and meet with the fossil remains of more and more ancient races of extinct animals, we find that many of them are actually intermediate between distinct groups of existing animals;" the ancient fishes, for example, present unmistakable reptilian traits, while the early reptilians combined also the characters of birds which had not yet appeared.

As regards the continuity of the course of life, Prof. Dana remarks: Geological history is like human history in this respect; time is one in its course, and all progress one in plan. . . . The germ of the period was long working onward in preceding time, before it finally came to its full development and stood forth as a characteristic of a new era of progress. . . . The beginning of an age will be in the midst of a preceding age; and the marks of the future, coming out to view, are to be regarded as prophetic of that future. The age of mammals was foreshadowed by the appearance of mammals long before in the course of the reptilian age, and the age of reptiles was prophesied in the types that lived in the earlier carboniferous age." The animal kingdom displays a unity of plan or a correlation of parts by which common principles are traced through the most disguising diversities of form, so that in aspect, structure, and functions the various tribes of animals pass into each other by slight and gradual transitions. The arm of a man, the fore limb of a quadruped, the wing of a bird, and the fin of a fish are homologous; that is, they contain the same essential parts modified in correspondence with the different circumstances of the animal; and so with the other organs.

Prof. Cope says: Every individual of every species of a given branch of the animal kingdom is composed of elements common to all, and the differences which are so radical in the higher grades are but the modifications of the same elemental parts." There are many cases of rudimentary and useless organs in animals and plants. During the development of embryos organs often develop to certain points, and are then reabsorbed without performing any function, although generally the partially developed organs are retained through life. Certain snakes have rudimentary hind legs hidden beneath the skin; the paddle of the seal has toes that still bear external nails; some of the smooth-skinned amphibia have scales buried under the dermal surface; rudi-mental teeth have been traced even in birds; and there are rudimentary eyes in cave fishes and rudimentary mamma) in men. Classification is an arrangement of living beings by related characters. In the earliest attempts the organic tribes were arranged in a serial order or a chain from the bottom to the top of the scale; but this has been abandoned, as also have those symmetrical systems which assumed that the characters of different groups are equivalents of each other.

The endeavor to thrust animals and plants into these artificial partitions is of the same nature as the endeavor to arrange them in a linear series; and it assumes a regularity which does not exist in nature. Classification now represents the animal kingdom as consisting of certain great sub-kingdoms very widely divergent, each made up of classes much less widely divergent, severally containing orders still less divergent, and so on with genera and species, like the branches of a growing tree; and the old method of classification, as Mr. Spencer remarks, involves exactly the difficulty which would meet the endeavor to classify the branches of a tree as branches of the first, second, third, fourth, and fifth orders; the difficulty, namely, that branches of intermediate degrees of composition exist." There is a remarkable analogy between the present distribution of animals in space over the earth and their past distribution in time as we trace their fossils in the successive geological formations. The larger groups, such as classes and orders, are generally spread over the whole earth, while smaller groups, such as families and genera, are commonly confined to limited districts; but when a group is restricted to one region, and is rich in the minor groups called species, it is almost invariably the case that the most closely allied species are found in the same locality or in closely adjoining localities.