cluding under
this term the warm periods between those of successive glaciations), and
in no one case does it appear that any modification of specific type has
occurred. This fact is particularly remarkable as regards leaves,
because on the one hand they are the organs of plants which are most
prone to vary, while on the other hand they are likewise the organs
which lend themselves most perfectly to the process of fossilization, so
that all details of their structure can be minutely observed in the
fossil state. Yet the interval since the glacial period, although not a
long one geologically speaking, is certainly what may be called an
appreciable portion of time in the history of Dicotyledonous plants
since their first appearance in the Cretaceous epoch. Again, if we
extend this kind of enquiry so as to include the world as a whole, a
number of other species of plants dating from the glacial epoch are
found to tell the same story--notwithstanding that, in the opinion of
Mr. Carruthers, they must all have undergone many changes of environment
while advancing before, and retreating after, successive glaciations in
different parts of the globe. Or, to quote his own words:--"The various
physical conditions which of necessity affected these {41} species in
their diffusion over such large areas of the earth's surface in the
course of, say, 250,000 years, should have led to the production of many
varieties; but the uniform testimony of the remains of this considerable
pre-glacial flora, as far as the materials admit of a comparison, is
that no appreciable change has taken place."

2. There is no appearance of generalized forms among the earliest plants
with which we are acquainted. For example, in the first dry land
flora--the Devonian--we have representatives of the _Filices_,
_Equisetaceae_, and _Lycopodiaceae_, all as highly specialized as their
living representatives, and exhibiting the differential characters of
these closely related groups. Moreover, these plants were even more
highly organized than their existing descendants in regard to their
vegetative structure, and in some cases also in regard to their
reproductive organs. So likewise the Gymnosperms of that time show in
their fossil state the same highly organized woody structure as their
living representatives.

3. Similarly, and more generally, the Dicotyledonous plants, which first
appear in the Cretaceous rocks, appear there suddenly, without any forms
leadin