1874.] Climate of the Glacial Period. 437 
glaciers—had occupied the ground for a long period, it re- 
treated, and peat mosses accumulated in low swampy spots ; 
but I dispute that there is any evidence of a warm climate. 
Cones of the Scotch and spruce firs, and leaves of the oak, 
the ash, and the yew, have been found in these deposits, 
and, as these are all of existing species, Prof. Heer has in- 
ferred that the climate was similar to that now experienced 
in Switzerland. In reality it may have been colder, for all 
these trees range to more northern latitudes. The bones of 
the large Mammalia found in the same deposits tell us 
nothing of the climate, or, at the most, do not throw any 
further light on the question than is derived from a study of 
the vegetable remains. All that is proved is, that towards 
the latter part of the glacial period the ice retreated, and 
after a long interval advanced again, and covered some great 
mosses that had accumulated during its retreat. We have 
had a similar event, though on a smaller scale, in historical 
times. M. Venetz has pointed out that before the tenth 
century the Swiss glaciers extended further than they now 
’ do, that then for four centuries they gradually melted back, 
and then again began slowly to advance, and have been ever 
since gradually regaining their lost territory. If this be so, 
they must have passed over surfaces on which vegetation 
grew during their retreat, and if these surfaces were again 
uncovered we might find leaves of existing Swiss trees in 
deposits between two sets of Moraine gravels, one of an 
earlier and one of a later date than when the trees 
flourished. 
Mr. Croll has himself advanced, as a crucial test of his 
theory, that as whilst one hemisphere was being glaciated 
the other was enjoying an almost tropical climate, and that 
as these conditions alternated several times during the 
period of greatest eccentricity, we ought to find proofs of 
the existence of these warm periods intercalated with those 
of greatest cold. And the evidence we require is, not that 
firs, oaks, and yews grew in Switzerland, as they do now, on 
moraines, during a temporary retreat of the ice, but of spe- 
cies that now live much further south, having then advanced 
far northwards. In faét, we want evidence, such as we 
have seen is so abundant in the Miocene strata, of a sub- 
tropical fauna and flora having flourished in Europe in inter- 
glacial times, and nothing less is satisfactory, according to 
this theory. The periods of greatest heat are as necessary 
a result of the theory as those of greatest cold, and they 
ought to occur alternately. 
I fully believe that if any one takes the trouble to read 
VOL. IV. (N.S.) 3K 
