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and Number of Animals in Geological Times. 287 
ent geological periods, and that during each, local faune 
may have existed with peculiar animals, &c. The ideas 
about characteristic fossils are still very crude, and nothing 
is more absurd than the complaints about unnecessary mul- 
tiplication of genera and species; as if both genera and spe- 
cies had not a natural existence, independent of the esti- 
mates of naturalists. It would be just as reasonable for as- 
tronomers to complain of the great number of stars, as for 
geologists to object to the investigations of zoologists, on the 
ground that they lead to the “making” of “too many 
species.” | 
The difficulty with reference to the identification of species 
is threefold: 1. Different species may be considered as iden- 
tical; 2. Specimens of the same species in different states of 
preservation, or of different age or sex, &c., may be consi- 
dered as distinct species ; or 3. The same species may have 
been described by different authors under different names, 
and their identity afterwards overlooked by later writers. 
Who does not see what amount of error may accrue from the 
indiscriminate use of materials which are not first submitted 
to a very critical revision in these different respects, not to 
speak of the general difficulty of agreeing upon the limits of 
specific differences. With regard to this last point, however, 
I would say that whosoever would only use in discussing 
general questions materials revised candidly with the same 
principles, could not fail to obtain at least uniform results. 
And when the results of investigations made upon materials 
corrected in different ways by different authors are compared 
with one another, if these differences are kept in view, the 
disagreement in the results would not be found so great as it 
might otherwise seem. The astronomers and physicists have 
long learned to correct their observations before using them, 
and to take into consideration what they call the personal equa- 
tion of different observers. Are we never to learn from them a 
lesson in the estimation of our respective investigations, and 
shall our facts for ever be used without being first corrected 
for all the possible causes of error and disagreement? As 
long as there are differences of opinion respecting the natural 
_ limits of genera and species, is it not absolutely necessary to 
