SOURCE OF EVOLUTION OF PROVINCIAL FAUNAS 603 
of this kind, but are such as are formed by the creeping out upon 
the low parts of the land of a film of the sea, as it were. The 
North and the Baltic seas, the Gulf of St. Lawrence and Hud- 
son’s Bay are adventitious examples. 
It is obvious that at a stage when the sea-shelves and the 
epicontinental seas were thus extending themselves the condi- 
tions for the expansional evolution of shallow-water marine life 
were signally favorable. In so far as land detritus is inimical to 
such life, an additional favoring factor is found in the reduction 
of the surface relief and the consequent diminution of the land 
wash. The seas at such stages were being not only extended 
but progressively clarified. A further incident of such stages is 
the free intercommunication of the life. All of the great con- 
tinents are at present connected by submerged portions of their 
platforms and appear to have been so united from the Cambrian 
times onward. Europe is connected with Greenland by a 
shallow tract, embracing Iceland, and Greenland, in turn, with 
the Arctic islands, and thence with the northeastern part of the 
American continent, constituting a northwest passage for Euro- 
pean shallow-water life. On the other hand, Asia is connected 
by a tract underlying Behring Sea and Straits, and by a broad 
belt along the border of the Arctic Ocean of unknown width, 
constituting a northeastern passage for Eurasian life. At times 
of base leveling there are broad sea-shelves girdling all of the 
continents, as well as internal epicontinental seas affording other 
connections; so that altogether the facilities for the migration 
and the intercommingling of the faunas are exceptionally pro- 
pitious. 
At the same time, as I have endeavored to show in another 
article in this number,’ the atmospheric and climatic conditions 
are uniform and favorable to the widest distribution of life. 
In such a period, therefore, is to be found the climax of 
conditions favorable to expansional evolution and to the devel- 
opment of a world-wide fauna of a composite and comprehensive 
tThe Effects of Great Limestone-forming Epochs upon the Constitution of the 
Atmosphere, pp. 609. 
