206 Professor Hoffmann on the Geological 
and Gay-Lussac to Vesuvius. We find, however, that compa- 
ratively little was then done by him in the way of special 
geognostical observations, for he was occupied to a much 
greater extent with the general consideration of the natural 
sciences connected with physical geography, and no subject 
of this description escaped him. No one has succeeded in so 
high a degree as he has done, in mastering and extending all 
the branches of that science. Soon after he had an opportu- 
nity of personally studying nature, he was attracted by Geo- 
gnosy, and more especially by some of the more important 
problems which are of the greatest moment for enabling us to 
form an opinion regarding the relations of our globe at the 
earlier periods of its existence. 
The subject which chiefly occupied his attention, was the 
change of climate which the earth must have undergone du- 
ring the periods that preceded our epoch. It is well known, 
that, in the different layers composing the crust of the globe, 
there occur remains of races of animals and vegetables which 
have successively existed and become extinct ; and that their 
forms correspond most nearly with the species of tropical 
climates, their whole organisation proving that there alone 
they could flourish. We do not yet know over what extent 
-of our earth these primeval organisms were distributed, but 
of this much we are certain, that many of them are met with 
in comparatively very high latitudes, in the temperate zone, 
and near the Polar regions, where they never could have 
flourished if the climate had not at that time been very simi- 
lar to that of the tropical zone at the present day. 
Various attempts have been made to explain this remark- 
able phenomenon in a manner consistent with nature; and, as 
the distribution of heat on the earth’s surface depends on the 
elevation which the sun attains above the horizon at each 
place, it has been assumed from an early period, in order to 
resolve the problem, that there formerly subsisted quite a differ- 
ent relation from the present one, as regards the position of the 
earth in reference to that of the sun. Instead of the plane of 
the ecliptic (the course of the earth round the sun) being, as 
at present, inclined at an angle of about Ook, to the plane of 
