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mammals which may have existed during this inter Secondary- 
Tertiary period, all trace is lost. The sedimentary strata of this 
interval of time, in which such remains might have heen preserved, 
are wholly unknown. Even the fresh-water deposits which may 
then have accumulated have entirely disappeared. 
At the commencement of the Tertiary era, however, the sea, in 
Europe, returned to a portion of the area which it had occupied 
during the Cretaceous period, and we have in addition to, and 
associated with the marine Eocene beds of Europe, fresh-water and 
estuarine deposits, from which have been obtained the remains of 
many quadrupeds. The tropical climate which obtained during 
this era, accompanied by its necessarily luxuriant vegetation, must 
have been singularly adapted to the existence and development of 
animal life. We know that in point of fact a numerous and 
varied mammalian fauna did exist, including representatives of 
most of our present families. 
There are at present known, according to Mr. Wallace, in 
the Old World, not including the Australian region, about 
240 genera, and more than 1400 species of land-inhabiting 
animals. Our acquaintance with the Eocene mammalia of the 
Old World is as yet obtained from a few localities in the 
South-East of England, in France, and in Switzerland only. We 
have no information whatever on this subject from the greater 
part of Europe, nor from the vast continents of Asia and Africa. 
In striking contrast with the above figures, it may be remarked, 
that from the lower Eocene beds, which have in England a 
maximum thickness of 750 feet, and include different horizons 
of marine, estuarine, and fresh-water deposits, and which doubtless 
represent an immense period, we have obtained less than a score 
species of mammals; about the same number from the deposits 
of the middle Eocene, which have about the same maximum 
thickness in this couutry as the lower division; and even from 
the rich gypsum strata of Montmartre only about fifty different 
forms. 
With reference to these latter deposits, which proved to Cuvier 
such a perfect mine of geological wealth, our imperfect acquaintance 
