go Seward: The Jurassic Flora of Yorkshire. 
stalwart Equisetums, which afford one of the few instances: 
of a genus still represented by several species in the British 
flora, which can claim a Jurassic ancestry. 
At first sight one might be tempted to infer from this hasty 
sketch that there is clear evidence of a tropical, or at least 
sub-tropical climate in Jurassic Europe. This would, perhaps, 
be a correct conclusion, but it is one which cannot be confi- 
dently made, so far at least, as the botanical evidence is- 
concerned. We must bear in mind the fact that among 
living plants very closely allied types, or even one and the same 
species, may flourish under widely different climatic conditions. 
as in the case of our familiar Bracken fern, which appears to- 
be equally at home on the Yorkshire moors, in Tasmania, in 
tropical Africa, Abyssinia, and elsewhere. The comparison 
of a past with a recent flora is bound up with numerous con- 
siderations in addition to those connected with the comparison 
of existing and extinct species. During the Rhaetic and 
Jurassic eras, and in the succeeding Cretaceous and Tertiary 
epochs, the genus Ginkgo was very widely distributed in Europe. 
As recently as the Lower Tertiary period it existed in what is 
now the west of Scotland, in a form hardly distinguishable 
from the Maiden-hair tree. Are we justified in assuming that 
the living species is a safe criterion as regards power of resistance 
or capabilities of life with which the family was endowed at 
the zenith of its vigour? Were it possible to learn from the 
Maiden-hair tree what vicissitudes its ancestors passed through 
since the days of the Jurassic period, we might hear of unequal 
competition and gradual migration from northern to southern 
latitudes, and of a retreat which brought the genus within a 
measurable distance of extinction. 
This brings me to an important point: admitting the in- 
sidious dangers of basing conclusions on negative evidence,. 
we may, I think, assert with confidence that the Flowering 
plants, that is the Monocotyledons and Dicotyledons which are: 
now the dominant class in the vegetable kingdom, were not 
represented in the Jurassic flora. It is true that a single speci- 
men of what looks like a Dicotyledonous leaf is known from 
the Stonesfield Slate, but this does not justify the statement 
that the flowering plants were then in existence. Even if this 
Stonesfield impression is that of a true Dicotyledon, there is 
not a particle of evidence pointing to the occurrence of the 
highest class of plants in the Jurassic flora as represented in 
the Yorkshire strata. This fact alone—that the Angiosperms. 
had not made their appearance, or to be on the safe side, if 
they had appeared they occupied a very subordinate position or 
were confined to habitats beyond the range of those agencies to 
which we owe the preservation of the Jurassic plants entombed 
in the Yorkshire beds—has an important bearing on the question 
Naturalist, 
