794 HISTORICAL GEOLOGY. 



and the vicinity of the Baltic, lias few species of Phylloceras and Lytoceras, 

 and very many of Harpoceras, Oppelia, Peltoceras, and Aspidoceras, and coral 

 reefs have great extent. The north Russian or Boreal province has in its 

 Jurassic rocks no species of Lytoceras, Phylloceras, and Haploceras, and no 

 coral reefs, while those of Cardioceras and Aucella are widely distributed. 

 On the other hand, the flora of the earliest part of the following Cretaceous 

 period in Greenland included an abundance of Cycads. 



Although the cold of the Atlantic and Pacific barriers of ISTorth America 

 was manifestly of little severity, it was enough for wide results in the 

 geographical distribution of species. 



The Mexican Gulf was a source of warm waters for southern and interior 

 North America, while at the same time the Arctic seas may have sent down 

 polar currents over its northwestern interior during the Triassic period. 

 The effects of the cold northwesterly currents of the Pacific border are 

 plainly seen in the many species peculiar to that coast, and prominently in 

 the Aucellae, which are related to the Siberian species. 



BIOLOGICAL CHANGES AND PROGRESS. 



Some of the Successional Lines. 



It is noteworthy that the new types of the Jura-Trias did not appear at 

 equable intervals successively along the era. They were rather evolvings 

 in its commencing part, the Triassic, the opening period of Mesozoic time. 

 The Triassic period is thus, after the Cambrian, which opened the Paleozoic, 

 the most eventful in the earth's biological history ; that is, the most pro- 

 ductive of great branchings in the higher departments of the Animal 

 Kingdom — the type of Mammals, that probably of Birds, and those of each 

 of the grand divisions of Eeptiles excepting such as had already appeared in 

 the Permian. This is true also of the modern, or nearly modern, style of 

 Orthopters, Neuropters, and Coleopters among Insects, as illustrated by 

 Scudder ; and the Lias completed the display of the system of Insects by the 

 introduction of the Dipters or Plies, and of Hymenopters as represented by 

 Ants and other families. It is to be admitted, however, that part of the 

 developments indicated by the relics in Triassic beds may date from the 

 Permian. The physical change of a purified atmosphere prepared the way 

 for terrestrial life ; and the preparation was essentially complete before the 

 close of the Permian. 



This crowding together of the origins of so many types in connection 

 with the barrenness of most Triassic regions makes it doubtful whether facts 

 illustrating the precursor lines will ever be fully made out. 



As regards the precursors of Mammals, their closer relation to Amphib- 

 ians than to Eeptiles is proved, as Huxley first pointed out, by the fact that 

 Amphibians and Mammals have two occipital condyles, and Eeptiles and 

 Birds but one ; and hence their derivation was almost certainly from some 

 Amphibian type, and not from a Eeptilian. The Monotremes (of which but 



