54 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
horizon, and species which when living were adjusted to different 
conditions of environment.® 
A fossil fauna is made up of the species which lived together under 
a common set of environmental conditions at the same time, and also 
of species which continued to be associated together for a greater or 
lesser length of time (they and their descendants), bearing the same 
relations to one another. It is this twofold extension which must be 
considered in dealing with the faunas of geological time, viz, their 
geographical distribution and their geological range. The geograph- 
ical distribution will indicate the limits of expansion of the fauna, 
determined, it is to be presumed, chiefly by conditions of exterior 
environment. The geological range will indicate the power of endur- 
ance of the whole fauna, and of the constituent species, in preserving- 
its integrity as a fauna, generation after generation, against the 
adverse changes of environment and against encroachment of other 
species. 
In order to get a definite concepl ion of a fossil fauna, it is necessary 
to ascertain what were the dominant species. Dominance is a rela- 
tive term, and implies an equilibrium among the several constituent 
members of the community. So complex a combination of forces is 
represented by a fauna that it can not be imagined that the relative 
dominance of the species of a fauna could be retained through any 
serious disturbance of the general conditions of life. A fauna thus 
characterized may be conceived of as keeping the equilibrium (once 
established among its constituent species) only so far geographically 
as the same conditions of environment prevail, and only so long geo- 
logically as it is able to continue breeding and living, at least in a 
metropolis of distribut ion whose conditions remain approximately con 
stant. A fauna once broken up in its biological equilibrium as a 
fauna must come to an end, however long thereafter individual species 
may persist. 
In order to appty these principles to the determination of the essen- 
tial characteristics of the Tropidoleptus fauna, two kinds of statistics 
were needed : 
(1) Statistics to show the dominant species of the fauna in its geo- 
graphical distribution over a considerable region of surface; and 
(2) Statistics to show the dominant species of a series of successive 
zones ranging through a considerable thickness of rocks in a single 
geographical section. 
CAYUGA LAKE SECTION. 
In order to provide a standard list of the fauna of what is called 
the Hamilton formation, from a typical section of the formation, I 
persuaded Mr. II. F. Cleland, already well equipped by his previous 
biological training, to make an exhaustive analysis of the Hamilton 
« Heterotopic is proposed to express this adjustment to diverse conditions of environment. See 
Bull. Geol. Soe. Am., Vol. XIV, 1893. 
