602 
SILURIA. 
[Chap. XX. 
no evidence whatever of high lands and detached mediterranean seas, or, 
if applied, it is the strongest evidence to show that the supposed primeval 
cliffs and lands were void of terrestrial vegetation. 
On what data, it may be asked, is founded the beautiful and rational 
theory of Lyell, which explains the successive changes of the climate of 
the earth? Is it not mainly dependent on those diversified evolutions, 
proceeding from beneath the surface, which have caused changes in the 
outline of former lands and seas, equivalent in extent, although different 
in position, to our present continents and oceans ? And if such a varied 
distribution of earth and water as the present had existed in the pristine 
periods that we have been considering, how could the same groups of 
animals, manifestly requiring the same conditions, the same temperature, 
and the same food have had so wide a diffusion ? 
Although it is quite true that specific distinctions are seen to have fre- 
quently prevailed in the fossils of the Lower Silurian rocks of countries 
situated at no great distance from each other, as explained in the Chapters 
which treat of the distribution of the fauna of that age, the fact is by no 
means antagonistic to my reasoning. Should it, for example, be even said 
that the variety in the distribution of Silurian species is as great as in the 
same areas of sea of the present day, I reply that it is not to species, but to 
the classes, families, and genera that, as a matter of fact, I appeal. If so 
many Trilobites, Cephalopods, Brachiopods, Cystideans, and Corals of analo- 
gous forms were spread out over areas at enormous distances, in the earlier 
primeval times, proving that oceans containing similar groups extended from 
China and the Himalayan Mountains, over Siberia and Russia to Western 
Europe, it is enough for me to feel assured that the various associates 
of the Calymene Blumenbachii, or any well-known Trilobite, must have 
required just the same temperature and surrounding media in whatever 
part of the world they lived. It is not because the land animals of 
Europe are dissimilar in species to those of Africa that the faunas of the 
two regions are so distinct, but because we have not among our European 
associates the same groups as those which live in hotter climes. I there- 
fore conceive that the fact of the wide diffusion of the same families 
and genera of Trilobites, Corals, and other fossils, however they may vary 
as to species, must have required an equably diffused temperature and 
similar conditions for their existence. Still more clearly is this infe- 
rence sustained in the Carboniferous era by the spread of the same- old 
vegetation, and often of the same species of Plants over half the globe. 
Having thus written in the last edition of this work, I am now, indeed, 
enabled to speak more conclusively on this point and to the same effect, 
thanks to the labours of Dr. Bigsby. The geologist who seeks to acquire 
a thorough acquaintance with the very wide diffusion of the whole Silurian 
Fauna must consult the forthcoming ' Thesaurus Siluricus ' of that inde- 
defatigable author, the result of many years of labour and research in both 
