and South America were upheaved. The western Alps rose to a height of 11,000 feet, and the Himalayas to a horizon 16,000 feet above the sea, while there were corresponding elevations in western North America and in the Rocky Mountain region.
The last great revolution, which, profound and widespread as it was in the Northern Hemisphere, did not apparently affect life and nature in the tropical zone, was the Glacial period. During this time there was, besides extensive migrations southward, and consequent modifications of species which could not resist the cold, a widespread extinction not only of numberless individuals, but of floras and faunas, a few forms becoming adapted to a circumpolar climate.
Biological Extinction Due to Competition. During all these changes, as the result of the struggle for existence, the competition between the outgoing and the incoming types and floras and faunas, there resulted vast biological changes, i.e. extinctions and re-creations.
In summing up the grand results of the Appalachian revolution and of the times immediately succeeding, Packard states that we should not lose sight of the fact that the changes in the earth's population were due to biological as well as geological and topographical factors. The process of extinction was favored and hastened by the incoming of more specialized forms, many of them being carnivorous and destructive. For example, nearly all fishes and reptiles live on other animals. The struggle for existence between those which became unadapted and useless in the new order of things went on more actively than at present. The process of extinction of the higher, more composite amphibians (the labyrinthodonts) was largely completed by the multitude of theromorphs and dinosaurs which overcame the colossal Cheirotherium, Mastodonsaurus, and their allies. Woodworth also states that “the exact cause of their decline is probably to be sought in the development of the more powerful reptiles.” The demise of the ornithosaurs or pterodactyls was assisted, says Packard, in two ways: Those with a feebler flight succumbed to the agile, tree-climbing dinosaurs; while the avian type, waxing stronger in numbers and powers of flight and exceeding in intelligence, exhausted the food-supply of volant insects, and drove their clumsier reptilian cousins to the wall, fairly starving them out; just as at the present day the birds give the bats scarcely a raison d'être.
At the close of the Jura-Trias period there was a widespread extinction of the peculiar coniferous plants of the Mesozoic, and they were succeeded by forests of deciduous trees of modern types. Vast forests of deciduous trees, such as the oak, sassafras, poplar, willow, maple, elm, beech, chestnut, and many others, as well as of conifers and palms, clothed the uplands, while in the jungles, on the plains, and in the openings of the forests, gay flowers bloomed. The flora must even then have been comparatively speaking, one of long existence, because highly differentiated composite plants, like the sunflower, occur in the Upper Cretaceous or Raritan clays of the New Jersey coast.
While the changes of level did not affect the abysses of the sea, the topography of the shallows and coast was materially modified, and to this was perhaps largely due the extinction of the ammonites and their allies.
In 1862 Wood more fully discussed this matter, and mentioned the same cause as suggested by Packard. “This disappearance,” says Wood, “of the Ammonitidæ, and preservation of the Nautilidæ, we may infer was due to the entire change which took place in the condition of the shores at the close of the Cretaceous period; and this change was so complete that such of the shore-followers as were unable to adapt themselves to it succumbed, while the others that adapted themselves to the change altered their specific characters altogether. The Nautilidæ having come into existence long prior to the introduction of the Ammonitidæ, and having also survived the destruction of the latter family, must have possessed in a remarkable degree a power of adapting themselves to altered conditions.” On the other hand, the dibranchiate cephalopods (cuttles or squids), living in deeper water, being ‘ocean-rangers,’ were quite independent of such geographical changes. Wood then goes on to say that the disappearance of the tetrabranchiate group affords a clew to that of the Mesozoic saurians, and also of cestraciont sharks, whose food probably consisted mainly of the tetrabranchiate cephalopods. “Now the disappearance of the Tetrabranchiata, of the cestracionts, and of the marine saurians, was contemporaneous; and we can hardly refuse to admit that such a triple destruction must have arisen either from some common cause or from these forms being successively dependent for existence upon each other.”
Woodworth suggests that mammalian life in the Mesozoic age was unfavorably affected by the nature of the peneplain of the Atlantic coast and by reptilian life.
“The weak marsupials or low mammals, which first appear in this country with Dromatherium in the tolerably high relief of the Trias, were apparently driven to the uplands by the more puissant and numerous reptilia of the peneplain. Their development seems also to have been retarded.” Again he says: “To sum up the faunal history of the Mesozoic alone, we have seen that pari passu 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 affairs 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 titled mammalia also were probably factors in terminating the life of the peneplains.”
After the placental mammals once became established, as the result of favorable geographical conditions of migration, isolation, and secondarily of competition, 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 mammals during Tertiary times which existed in America north of Mexico have been already described, where at present there are scarcely more than three hundred. This is an example of the amount of extinction which
