102 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
sediments the corporate integrity of the fauna is ascertained by 
observing the continuance of dominance of the dominant species. 
It was found that at an}^ particular stage of the fauna certain species 
were dominant, as indicated by their abundance in the particular 
faunule. The relative abundance of the species gave a means of 
estimating the particular adjustment of the species to one another 
at the particular time and in the particular environment of the 
faunule. The temporal equilibrium was not found to be preserved 
for much thickness of strata, nor, when studied geographically, for 
much distance of distribution; such a faunule with its exact combi- 
nation and proportion is both temporary and local, and constitutes 
the type of a single faunal unit — i. e., a monobion, and its time limit 
is 1 lie hemera. 
Slight change of conditions, not sufficient to effect permanent 
change in the specific characters of the species, either coincident with 
passage of time or with change of position, may disturb the equilib- 
rium, and the effect of the change is exhibited primarily in the differ- 
ent relations of abundance or rarity of the constituent species. 
The difference in these respects observed upon comparing the suc- 
cessive faunules is found to consist in a change in their relative 
dominance as constituent species, and rarely in the entire absence of 
any of the more common species, when imperfection of the collection 
is fairly taken into account. Certainly the fads indicate that there 
was no extinction of the species, for they came in again at successive 
places higher up in the column of strata. 
To ascertain, then, the real character of the fauna as a corporate 
whole, in terms of species, it is necessary to ascertain what species 
are sufficiently dominant to overcome the lesser changes of conditions, 
and to hold their preeminence, continuously, coincident with succes- 
sion of faunules as recorded in the geological column of a single sec- 
tion, and coincident with changes of conditions as indicated by the 
faunules taken from separate geographical localities. 
The species Avhich appear most frequently in sample faunules, rep- 
resenting geological succession and geographical distribution, may 
hence be regarded as the most characteristic representatives of the 
fauna for the total period of time during which it has preserved its 
faunal integrity and over the region in which it was normally 
adjusted to live. The presence of any large number of such domi- 
nant species of a fauna may be safely regarded as indicative of the 
epoch in which the fauna was dominant, and which may be appro- 
priately designated as the epoch of that fauna. 
This would be a reasonable conclusion even in case species of a 
fauna which in general succeeds it were present and associated with 
it in force. The argument for this conclusion is that the fauna can 
not be regarded as having ceased its existence as a fauna, so long 
as in a single faunule, anywhere, the species which have all along 
