75 
found in the ‘ Quarterly Journal of the Geological Society of London, 
1846,’ by Professor Ansted ; and I think I cannot do geology better 
service than by making some extracts from it. 
“Tt is one of the most elementary facts of geology, that there are 
certain groups of species found fossil in the various sets of strata of 
which the earth’s crust is made up, and that these demonstrate the 
existence of something like a series of distinct faunas... ... . The 
comparison of such faunas sometimes presents very important results, 
by generalising from which certain supposed laws have been arrived at, 
which it is assumed have governed the succession of organized beings. 
Possibly these have been too hastily assumed, and the importance of 
some phenomena overrated ; but at any rate such generalizations have 
been useful, and they have greatly tended to advance paleontological 
science.” 
These views have been very generally admitted in reference to the 
four great geological periods, viz. the Diluvial, Tertiary, Secondary, 
and Primary ; but recent investigations have gone further, and pointed 
out, that these large divisions may also be separated into distinct 
faunas, each having characteristic fossils. The Cretaceous formation, 
for instance, has been divided into the upper and lower greensand, the 
gault, and chalk, and as a general law, with much propriety and cor- 
rectness. 
‘ This law is true for all classes of animals, but is differently exhi- 
bited in each, the groups first introduced having, it would seem, under- 
gone the smallest amount of change, and the converse. Thus if we 
compare the mollusca and mammalia, we shall find that the former, 
which existed at the earliest period, have hardly changed their form 
since the close of the Cretaceous epoch, and that the shells of the 
tertiary period are generically the same as those now existing ; while 
the mammalia, though only recently troduced, have been subject to 
many changes, and have required the introduction of new and import- 
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