Williams] ANIMAL AND PLANT AGGREGATES. 15 
lie speaks of Paleozoic faunas, Carboniferous floras, the fauna of the 
Trenton or of the Cambrian or of the Eocene. The basis of aggrega- 
tion in these cases is the fact of living at the same time, or period of 
time, in the earth's history; or, to speak more abstractly, the geolog- 
ical range of the organisms. The Eocene fauna includes all the ani- 
mals, of whatever descent or of whatever zoological rank, existing in 
all kinds of environments, of which fossil remains are known occurring 
in the Eocene formations of the whole world. As at present defined the 
term Eocene is applied to formations of ditferentlithological kinds, out- 
cropping in various parts of the world, the only final test of the Eocene 
age of which is the uniformity of the faunas. Hence it is evident that 
the assumption is made that the whole life of the globe for each period 
of time is in a marked degree alike for like conditions of environ- 
ment. But this conclusion is true only when the qualifying phrase 
in a marked degree is kept in mind, for a comparison of the faunas 
and floras from different parts of the earth now living shows them to 
differ, though living under like conditions of environment. 
Students of geographical distribution have shown that in distant 
parts of the same ocean the species are widely divergent, as much 
difference existing between the marine faunas of the southern and 
northern temperate zones as between the faunas of two successive for- 
mations of a continuous geological section. It is evident from this 
observation that discussions of the time relations of fossils must treat 
not only of the genetic affinity of the forms making up a fauna, but 
of the geographical distribution and of the geological range of the 
species concerned. 
While species, genus, etc., have been adopted as terms to express 
genetic affinity of the organic aggregates under consideration, fauna 
and flora are general terms used to indicate aggregates of animals or 
plants associated on the basis of their geographical distribution (or 
adaptation to similar conditions of environment) and their geological 
range (or place in the evolutional history of the total life of the globe). 
It is no longer internal structure but external conditions which 
determine these latter aggregations 
In discussing fossil aggregates of organisms we have to consider, 
therefore, this threefold relationship they bear, viz, (a) to zoological 
and botanical classification, (b) to geographical distribution, and (c) 
to geological range. 
ZOOLOGICAL AND BOTANICAL CLASSIFICATION. 
The first kind of relationship is expressed by the internal structure 
possessed by the organisms themselves; hence the definition of an 
aggregate of this kind is in terms of morphological characters, and 
its classification is based upon the rank (the taxonomic rank) of these 
characters, which is indicated by the technical name of the species or 
genus or order to which the individual organism is said to belong. 
