318 L, Agassiz on Animals in Geological Times. 
upper secondary formations, as great as in any tropical region? 
ave we not sufficient indications among the tertiaries to be 
justified in expecting that they also will turn out to be more nu- 
merous than they are now known to be 
The class of Birds seems to form au exception in this view. 
But there seems to be particular reason why the bones of birds 
should be more liable to destruction and decomposition than those 
of other vertebrata. And whoever has traced the discoveries 
made recently among the fossils of this class, will certainly not 
insist upon a supposed scarcity of birds in former periods, but 
rather be inclined to admit that the limited number now known 
is to be ascribed to the deficiency of our knowledge rather than 
to a want of these animals in earlier formations, indications 
their presence having been ascertained for several tertiary forma- 
tions, for cretaceous deposits, and even for deposits belonging to 
periods older than the chalk. 
Fossil Mammalia are comparatively too well known to call for 
many remarks, after what has been said above. Let us only re- 
member that the number of fossil species found in Brazil alone 
equals the whole number of Mammalia known to live at preset 
in that country; that the fossil Mammalia of New Holland com- 
pare already favorably with the living species of that continent ;, 
and that the locality of Montmartre alone bas yielded as many 
granted that the great variety of types which occur at any latet 
riods has arisen from a successive differentiation of a few still 
earlier types, it should be shown that in reality in former pe 
the types are fewer and less diversified ; and we have now shown 
real 
types of the animal kingdom, I need not therefore repeat here 
what may be gathered from the diagram at the head of the we 
ological Text Book I have published jointly with Dr. Gould. 
shall limit myself to a few more general remarks upon the spe- 
cial difficulties involved in a more thorough investigation of th 
