212 +. ~~ Professor Hoffmann on the Geological 
nearly that which prevails at the present day in these regions 
The same thing is indicated by the beds of shells and corals, 
and by the vegetable forms, which are found in the newest 
strata of the tertiary series ; for even their forms belong to a 
tropical world, and they are preserved with as much freshness 
and integrity in their most delicate portions, as if they had 
been destroyed at the period of their fullest perfection, by a 
change of climate which they could not resist, at the last re- 
volution which has affected our earth. 
This suddenness of the change is not taken into account by 
Humboldt, in his suppositions regarding the causes of the al- 
teration of temperature, According to his view, a successive 
diminution of the previously existing higher temperature, pro- 
duced by the gradual disappearance of the quantity of heat 
communicated to the atmosphere, must have taken place, and 
the products of a tropical climate must gradually have been 
incommoded and enfeebled, and have been replaced by such 
products as correspond with the present arrangement of the 
climate. The largest portion of the former would have been 
extinct, and, at least in so far as regards the more delicate 
forms, destroyed, long before they were covered by the super- 
imposed strata. But there is no trace in the strata of the crust 
of the globe of such a gradual diminution of fitness for cer- 
tain forms, or of such a mingling of the products of the tropi- 
cal climate with those of the present distribution of the heat 
of the surface of the earth. Both races, that of the newest, 
and that of the immediately preceding ancient period, are 
sharply separated from each other ; and hence, also, one of the 
greatest naturalists of our time who has devoted himself to 
this subject, Cuvier,* has laid especial weight on this sudden 
occurrence of the last of the revolutions which have taken 
place on the surface of our earth. We thus at once perceive, 
that all attempts to explain this wonderful fact have been un- 
: satisfactory, and that it must still be reserved for the future, 
as a problem of extreme importance, in the knowledge of the 
various events in the formation of our earth’s crust. 
In the view promulgated by Humboldt, we find the first 
* Discours Préliminaire to the Recherches sur les Ossemens Fossiles. 
