C.—GEOLOGY 77 
to the ideas of the time, have grown under tropical conditions. There 
is no a priori reason, however, why this should be the case. Luxuriant 
growth is not a feature exclusively tropical, it is more a question of water 
supply at the proper time. Still less can we accept the contention when 
we find that the luxuriant Glossopteris flora, co-existing with the northern 
one, was associated with glacial conditions ; the glaciers, in places, coming 
_ down to sea level. ‘Tropical conditions in the northern hemisphere at 
7s 
the same time as glacial conditions in the southern, simply could not be 
brought into unison. The explanation by Wegener’s hypothesis of 
continental drift is an easy way out of the impasse ; but this is only one 
case among several, and even Wegener would hardly have accepted the 
wanderings of his continents that would be required to explain all the 
known occurrences. 
Again, the presence of plants, whose living relations inhabit tropical 
lands, in the early Mesozoic rocks in Greenland is no proof of tropical con- 
ditions in Greenland at that time, for a genus of plants may have some 
species capable of enduring more rigorous conditions than others, and, 
moreover, there may be a progressive decline in the power of certain 
genera to withstand any but tropical conditions as time goes on. Prof. 
Seward further pointed out that the present diversity in floras is largely 
due to the great preponderance of flowering plants, and that the apparent 
uniformity in past floras was therefore illusionary. 
Then we must never forget that fossil plants are almost always ‘ form’ 
genera and ‘ form’ species, and that, even within a single genus to-day, 
we may have some species confined to tropical regions and others that 
tolerate a colder climate. Consequently, a specimen that cannot be 
accurately proved as identical with a living species may be little or no 
‘use as a climatic index. 
Of course when specimen after specimen points in one and the same 
direction, so far as probable climatic characters are concerned, then we 
cannot disregard that indication, and when such indications can be 
confirmed by collateral phenomena, such as an apparent sub-tropical 
flora and entire lack of tundra features in a region now in the tundra belt, 
the conclusion is inevitable that climatic change has taken place. 
Taking everything we know into consideration, the general consensus 
of opinion is that plants do afford an index of climatic changes, and that 
these changes have been very considerable in past times. 
Can we explain those changes, and can we obtain an explanation that 
will not conflict with other evidence ? 
In the past few years interest in the problem has been awakened by 
Wegener’s theory of continental drift, or, perhaps better, the modification 
of Wegener’s hypothesis suggested by the late Prof. Joly of Dublin. Yet 
it must be remembered that Lyell and Darwin © were considering the 
problem seventy years ago, so that it is no new geological puzzle. 
52 Lyell’s Letters. Letter to Darwin, March 10, 1866. ‘I have been doing my 
best to do justice to the astronomical causes of former changes of climate, as I 
know you will see in my new edition [Principles of Geology], but I am more 
than ever convinced that the geographical changes are, as I always maintained, 
the principal and not the subsidiary ones.’ 
