504 STUDIES FOR STUDENTS 
ture conditions upon the earth would be far more nearly equa- 
ble than they are today, so that the same species could live both 
in the tropics and in the polar regions, as the distribution of the 
fossils indicates to have been the case, Under such conditions 
the shallow-water marine organisms would have the most favor- 
able opportunity for intercommunication with all parts of the 
world, and there would be developed just such a cosmopolitan 
fauna as we know existed in Silurian time. 
The Devonian period gives us a good illustration of the pro- 
vincial typeof faunas. During this period the shallow-water marine 
faunas exhibit a great diversity in various parts of the world. The 
Devonian faunas in New York, or even in southern Illinois, have 
less in common with those of Iowa, than have the Silurian faunas 
of the interior of North America with those of Europe or even 
with those of Australia. The Devonian faunas of Europe are also 
different from the American, and, furthermore, they differ among 
themselves. In South America and other parts of the earth there 
are still other types of Devonian life. The evidence afforded by the 
distribution of species and genera indicates without a doubt that 
the provincial development of faunas was one of the most con- 
spicuous characteristics of the period. This type of develop- 
ment was, of course, brought about by the greater or less isola- 
tion of great shallow-water tracts in different parts of the world, 
in each of which the evolution of the life progressed along its 
own peculiar lines, molded by the particular environmental con- 
ditions which obtained in each tract or province. Because of 
the more or less complete isolation of these various provinces, 
and the absence of shallow-water paths along which intercom- 
munication between distant parts of the earth was possible, the 
development of a cosmopolitan fauna such as existed in Silurian 
time was out of the question. 
In addition to the provincial development of the Devonian 
faunas, the life history of the period is complicated in a still 
further degree by reason of the continuous changes in the 
See “A Systematic Source of Evolution of Provincial Faunas,” by T. C. 
Chamberlin, Jour. GEOL., Vol. VII, p. 597. 
