Williams] FAUNAL AGGREGATES. 29 
ment, a un moment quelconque des temps geologiques, comme cela se 
oasse encore au temps actuel. " a 
Renevier, although distinguishing between the duration of time of 
the formation and the means of recognizing that duration, viz, the 
different faunas which are found in the different kinds of deposits, 
still makes the time division synonymous with the duration of the 
work of producing the formation, not the duration of the living of 
the organisms whose remains are seen in the fossils. 
A fossil fauna may characterize a formation without having its 
limits (chronological) determined by the beginning or cessation of 
deposition of sediments making up the formation. In fact, a fauna 
which appears in full force at the base of a formation must have 
existed somewhere for a long geological period of time before the 
specimen of it (the faunule) which, occupies the lower layers of the 
formation was buried, or else we are forced to assume that it was 
'suddenly created on the spot. 
If this proposition be true, and I think no modern paleontologist 
will question it, the common methods of correlating the time equiva- 
lency of formations by the likeness of their fossil faunas is inaccurate 
at least by such a length of time as would be required for the estab- 
lishment of that coadaptation of the species which characterizes the 
fauna during its whole expression in the given formation. The 
change of faunas in successive formations which on other grounds 
may reasonably be supposed to represent continuous sedimentation, 
frequently is very abrupt and complete. It is only occasionally that 
a gradual transition of the species is actually recorded in the succes- 
sive beds of a continous rock section. And within the limits of a 
stratified formation, as generally recognized, the same species prevail, 
not always presenting the same relations of abundance throughout, 
but the same species, and each one with less amount of variation than 
is expressed by the representatives across the line by which the for- 
mations are distinguished. 
What takes place with the living organisms during the transition 
of one formation to another has not been thoroughly observed or dis- 
cussed. This failure of knowledge is certainly in some measure due 
to the practice of assuming that the time duration of the fauna is 
synonymous with the time duration of the formation which in some 
particular locality contains it. 
In order to differentiate the fauna from the formation, it is needful 
to observe the characters which pertain to faunas and not to formations. 
A fauna is an association of species which for some reasons natu- 
rally live together. It is described in terms of species, genera, orders, 
etc., and not by formations or localities in which it temporarily lived. 
A faunule is a local sample of the fauna. The fauna at a particular 
aLoc. cit., p. 528. 
