regions of the shallow sea between the Aleutian Isles and Behring Strait. Through this gate-way came an invasion from Asia, including probably the mammoth. With this invasion probably came also man. It seems probable, therefore, that the earliest remains of man in America will be found on the Pacific coast.

Also the great Pliocene lake, which stretched from near the shores of the Gulf far into British America, and possibly into arctic regions, and formed a more or less complete barrier to the mammalian fauna east and west, was abolished by upheaval, and free communication was established. It is impossible that all these changes of climate and all these migrations, partly enforced by changes of climate and partly permitted by removal of barriers, and in this latter case especially attended with the fiercest struggle for life, should not produce rapid and profound changes in the mammalian fauna.

In Europe the process has been more accurately studied and is better known. In Quaternary times at least four different mammalian faunæ struggled for mastery on European soil. (1.) The Pliocene autochthones. (2.) Invasions from Africa by opening of gate-ways through the Mediterranean: one by way of Italy, Sicily, and Malta, and one by Gibraltar, both of which have been again closed. (3.) Invasions from Asia, by removal of a great sea barrier connecting the Black and Caspian seas with the Arctic Ocean. This gate-way has remained open ever since. (4.) Invasions from arctic regions, enforced by changes of climate. Probably more than one such invasion took place; certainly, one occurred during the second Glacial epoch. The final result of all these climatic changes and these struggles for mastery was that the Pliocene autocthones, adapted to a more genial climate, were mostly destroyed or else driven southward with some change into Africa: the African invaders were driven back also into Africa, and with some Pliocene autocthones isolated there by subsidence in the Mediterranean region closing the southern gateways, and still exist there under slightly modified forms; the Arctic invaders were again driven northward by return of more genial climate, and there exist to this day; while the Asiatics remained masters of the field, though greatly modified by the conflict. Or perhaps, more accurately, we might say that the existing European mammalian fauna is a resultant of all these factors, but the controlling factor is the Asiatic. With the Asiatic invasion came man, and was a prime agent in determining the final result.

Thus, regarding the Tertiary and the Present as consecutive eras, and the Quaternary as the transition or critical period between, then, if the record of this period had been lost, corresponding with the unconformity here found, we should have had here an enormous and apparently sudden change of mammalian species. Yet this change of fauna, as great as it is, is not to be compared with that which occurred between the Archæan and Paleozoic, or between the Palæozoic and Mesozoic, or even that between the Mesozoic and Canozoic; for the change during the Quaternary is mostly confined to species of the higher mammals, while the change during previous critical periods extended to species of all grades, and not only to species, but to genera, families, and even orders. We conclude, therefore, that the previous critical periods or lost intervals were far longer than the whole Quaternary; or else that the rate of evolution was far more rapid in these earlier times.

To sum up, then, in a few words, the general formal laws of evolution-change throughout the whole history of the earth: (1.) Gradual, very slow changes of form everywhere under the influence of all the factors of change, known and unknown: for example, pressure of changing physical conditions whether modifying the individual (certainly one factor), or selecting the fittest offspring (certainly another factor); improvement of organs by use and the improvement inherited (certainly a third factor), and perhaps still other factors yet unknown. This general evolution by itself considered would produce similar changes everywhere, and therefore would produce geological faunæ, but not geographical diversity. Determination of a geological horizon would in this case be easy, because fossil species would be everywhere identical.

(2.) Changes in different places and under different physical conditions, taking different directions and advancing at different rates, give rise to geographical fauna. This, if there were nothing more, would produce far greater geographical diversity and more complete localization of faunæ and floræ than now exists, so great that the determination of a geological horizon would be impossible.

-

(3.) The force of change resisted by heredity, in some species and genera more than in others, determines paroxysms of more rapid movement of general evolution, affecting sometimes species, sometimes genera or families. The sudden appearance of species, genera, families, etc., in quiet times is thus accounted for.

(4.) During critical periods, oscillations of the crust, with rapid changes of physical geography and climate, determine a more rapid rate of change in all forms: first, by greater pressure of physical conditions; and, second, by migrations partly enforced by the changes of climate and partly permitted by removal of barriers, and the consequent invasion of one fauna and flora by another and severe struggle for mastery. This would tend to equalize again the extreme diversity caused by the second law; but the effect would be more marked in the case of animals than plants, because voluntary migrations are possible only in this kingdom. Hence it follows that a geological horizon is far better determined by the fauna than by the flora.

III. Historic Value of the Present Time. Most geologists regard the Present as one of the minor subdivisions of the Canozoic era, or even of the Quaternary period. More commonly the Quaternary and Present are united as one age the age of man

- of the Cenozoic era. The Cenozoic is thus divided into two ages the age of mammals commencing with the Tertiary, and the age of man commencing with the Quaternary; and the Quaternary subdivided into several epochs, the last of which is the Present or Recent. But if the views above expressed in regard to critical periods be correct, then the Present ought not to be connected with the Quaternary as one age, nor even with the Cenozoic as one era, but is itself justly entitled to rank as one of the primary divisions of time, as one of the great eras separated like all the other eras by a critical period; less distinct it may be, at least as yet, in species than the others, the inaugurating change less profound, the interval less long, but dignified by the appearance of man as the dominant agent of change, and therefore well entitled to the name Psychozoic sometimes given it. The geological importance of the appearance of man is not due only or chiefly to his transcendent dignity, but to his importance as an agent which has already very greatly, and must hereafter still more profoundly, modify the whole fauna and flora of the earth. It is true that man first appeared in the Quaternary, but he had not yet established his supremacy; he was still fighting for mastery. With the establishment of his supremacy the reign of man commenced. An age is properly characterized by the culmination, not the first appearance, of a dominant class. As fishes existed before the age of fishes, reptiles before the age of reptiles, and mammals before the age of mammals, so man also appeared before the age of man.

We therefore regard the Cenozoic and Psychozoic as two consecutive eras, and the Quaternary as the critical, revolutionary, or transitional period between. But since the record of this last critical period is not lost, and we must place it somewhere, it seems best to place it with the Cenozoic era and the mammalian age, and to commence the Psychozoic era and age of man with the completed supremacy of man, that is, with the Present epoch.

BERKELEY, CALIFORNIA, March 15, 1877.

RECENT LITERATURE.

BREHM'S ANIMAL LIFE.1. A second edition of Brehm's well-known German work on the animal kingdom is now in course of publication, to be issued in about a hundred parts, published weekly or fortnightly, forming a series of volumes of unusual attractiveness and interest. The author tells us that it is really a new work under an old title, having been rewritten and enlarged. While the first volume of the first series treats of the mammals from the apes down to the family of dogs, succeeding volumes will treat of the other mammals and of the reptiles and fishes. These will be written by Dr. A. E. Brehm, the wellknown naturalist, while those on the articulated animals will be written by Prof. E. L. Taschenberg, of Halle, and the mollusks will be treated of by Prof. Oscar Schmidt, of Strasburg. What provision is to be made for the other invertebrate animals is not yet announced.

The work is of a general nature, not designed for the special zoölogical student or for children, but for those who wish to gain a knowledge of the principal forms of animal life, their habits and distribution. There are no troublesome, perplexing anatomical or embryological details, save wood-cuts of skeletons, in word or picture, no foot-notes, and the style is easy, sprightly, and often colloquial. It is apparently a popular work in the best sense of the word, and should be well patronized in this country, if for no other reason than for the wealth of wood-cuts, both full-page and textual, which alone, to those ignorant of German, would make it of lasting value. The illustrations are nearly all new to us, and in very many, we suppose most, cases are drawn from life by such artists as R. Kretschmer, G. Mutzel, and E. Schmidt, with the greatest apparent fidelity.

The plan of the work is excellent. After au introductory chapter on life in its totality, the apes are described, — man, the type of the first suborder of Primates, being referred to only incidentally in comparison with 1 Brehm's Thierleben. Allgemeine Kund, des Thierreichs. Grosse Ausgabe. Zweite umgearbeitete und vermehrte Auflage. Erste Abtheilung, Säugetiere. Erster Band. Leipzig Verlag des bibliographischen Institute. 1876. 8vo, pp. 706. New York: B. Westermann & Co. 40 cents a part; 12 parts to a volume.

the apes, which represent the second suborder of Primates. Then follows a general "popular" account of the forms of apes, their geographical distribution, dwelling-places, food, motions, social life, language, reproduction, education, rearing of young, diseases, life in confinement, and of the apes figured on the Egyptian temples; then succeeds a more special account of the gorilla, the chimpanzee, and the tschego (Anthropopi

thecus tschego), and orang, with the lower forms. In this comprehensive and, we may readily believe from the high reputation of the author, thorough manner, the different groups of animals are treated. A sample of the elaborate nature of the wood-cuts is afforded in the two accompanying views of the head of a five-year old tschego, in the Dresden Zoological Garden, which was brought from the Loango coast.

(FIG. 59.) TSCHEGO APE.