"The weak marsupials or low mammals, which first appear in this country with Dromatherium in the tolerably high relief of the Tries, were apparently driven to the uplands by the more puissant and numerous reptilia of the peneplain. Their development seems also to have been re tarded." Again he says: "To sum up the fennel history of the Mesozoic alone, we have seen that yari yassu with the creation of broad lowlands there was brought on to the stage a remarkable production of reptiles, a characteristic lowland life; and we note that the humble mammalia were excluded from the peneplain or held back in their development, so far as we know them by actual remains, during this condition of af fairs until the very highest Cretaceous. At the close of the Mesozoic, the area of the peneplain was uplifted and there came into it the new life. Not only the changed geographic conditions, but the better fitted mammalia also were probably factors in terminating the life of the penepla ins." After the placental nmininals once beeame es tablished, as the result of favorable geographical conditions of migration, isolation, and second arily of eompetition. the evolution as well as the elimination of forms, as is well known, went on most rapidly. Remains of over two thousand species of extinct mainnifils during Tertiary times which existed in America north of Mexico have been already described, where at present there are scarcely mere than three hundred. This is an example of the amount of extinction which went on in a single class of animals. There must have been corresponding rates of extinction in the case of birds, fishes, and insects.
The rapid summary we have given of the suc cessive changes and revolutions in the earth's history, and the fact that they are accompanied or followed by the process of the extinction of the unadapted, and their replacement by the more specialized and better adapted, show that there is between these two sets of phenomena a relation of cause and effect. The subject is further illustrated by the extinction of life in South America.
The Andean plateau during the Quaternary period was paroxysmally elevated into the air some 12,000 feet. Packard calls attention to the possible results of such an enormous up heaval on the plants and animals of this region. Before and at the time this movement began, when the land was 12,000 feet lower than now, the Atlantic trade winds which now cross Brazil, impinge upon the Andes, and drop their moisture on the eastern slopes alone, then favored as well the western slopes and Pacific coast. The tropical flora and fauna now confined to the neighborhood of Guayaquil on the coast of Peru then probably spread over Bolivia, Ecuador, Peru, and Chile to Patagonia. At Riobamba, altitude 9200 feet, the climate and vegetation are temperate; here occur bones of the mastodon, horse, deer, and llama—animals which may have lived in a tem perate climate. But was not their extinction, and that of the colossal sloths, armadillos, and other animals of the pampas largely due to a change of climate resulting from the elevation of the Andean plateau? As the land gradually rose, the atmosphere would become more rarefied and insupportable to tropical life; the animals and plants would either seek lower levels or undergo extinction, or in certain cases become modified into species suited to a temperate climate. As
the plateau rose still higher, the air would he roine too cold and rarefied for even the mastodon and horse. Gradually an alpine zone became es tablished, and finally the higher peaks of the Andes, at an elevation of 15,000 feet, became mantled with perennial snow, and on Chimborazo glaciers established themselves. We thus see how, within Quaternary times, temperate and alpine zones became established over the vast Andean plateau, originally, perhaps at the end of the Pliocene, a plateau of the third order, clothed with vast forests like those of Brazil and Venezuela.
Another, but more local cause of extinction, is seen in Great Salt Lake, Utah. Formerly this was a vast fresh-water lake, abounding in fish, insects, mollusks, and plants. When it was by elevation of the lake-basin transformed into a brine-pool, all life was extinguishO, except a shrimp, a single species of fly, and an alga. So with deserts; when they are formed life is re duced to a relatively small proportion.
That there is a limit to the age of species as well as to individuals almost goes without saying. As there is in each individual a youth, manhood, and old age, so species and orders rise, culminate, -and decline, and nations have risen, reached a maximum of development, and then decayed. The -causes, however complex, are, in case of plants and apparently physical: they are general and pervasive in their effects, and have been in operation since life began; there have been critical pi•riods in paleontological as well as geological history, and periods of rapid and widespread extin•tion as well as a continual, progressive dying-out of isolated species. Such extinction was, so to speak, a biological neces sity, for otherwise there would have been no progress, no evolution of higher types.
Min. Origin of Species (London, 6th ed., 1882) ; Searles V. Wood, "On the Eurin and Distribution of the Land Tracts during the Secondary and Tertiary Period•, re spectively. and on the Effects upon Animal Life which great changes in geographieal configuration have probably in Philosophical Mugu zinc, xxiii., p. 161 (Edinburgh, 1862) ; A. S. Packard, "Geological Extinction and some of its apparent causes," in American. Naturalist, xx. 29 40 (Philadelphia, ,January, 1880) ; "A Half-Cen tury of Evolution, with special reference to the effects of Geological Changes on Animal Life," in Proceedings of the American Association, for the Adrancement of Science (Boston, 1898) ; B. Woodworth, "Base-Levelling and Organic Evolu tion," America,' Geologist, xiv. (October, 1894).