SILURIAN FAUNA INTERPRETED EPICONTINENTALLY 695 
in a time sense, the Clinton and Niagara divisions of the Silurian 
must be considered as a unit, the stratigraphic distinctions 
between the two being of but local significance. 
In like manner the Clinton and Niagara faunas must be con- 
sidered asa unit. To be sure there are species which in any 
given area are known only in the Clinton strata, and others 
which are restricted to the Niagara limestones; there are also 
species restricted to a single stratum of either one division or 
the other. From the very nature of the case this would be 
expected, because all organisms are not so constituted as to be 
able to adjust themselves to all conditions of environment. 
There are always sure to be local adaptations in any general 
fauna, to the varying local conditions both in time and space. 
And so we must look upon the general Silurian fauna of America, 
not as constituted of two sharply defined faunal divisions, the 
Clinton and the Niagara, but as being one composite faunal unit 
composed of numerous faunule, adjusted to a great variety of 
local environments. 
The same general Silurian fauna which occupied so large a 
portion of the North American continent, also was present in 
other parts of the world. In Europe it is recognized with its 
local adaptations in England, in the island of Gotland, in Russia, 
in Bohemia and elsewhere. Many species are common to the 
Silurian beds of England and North America, and there are like 
relations between the faunas in America and other- parts of 
Europe. In other parts of the world this same general fauna 
has been found. As far away as New Zealand,' a Silurian fauna 
has been studied, in which there are several species common 
to Europe and North America. These facts show that there 
must have been intercommunication between the Silurian seas of 
different parts of the world, and means of intermigration for the 
organisms which inhabited them. 
Although it has long been recognized that some means of 
intercommunication between Europe and the interior of North 
America must have existed during Silurian time, the pathway 
* Quart. Journ Geol. Soc. Lond., Vol. XLI, p. 199 (1885). 
