Chap. XX.] 
GENERAL SPREAD OF PRIMEVAL LIFE. 
501 
that supplied the elements for the construction of great coal-fields. (See 
Sketch, p. 286.) These phenomena assure us, therefore, that a very great 
portion, if not indeed the whole surface of the earth, enjoyed at that time 
an equable and warmer climate. 
Believing, as I do, with some geologists, that this former temperature of 
the earth was, in a measure, probably increased by the radiation of its inter- 
nal heat, independently of solar action, other physical as well as zoological 
phenomena lead me further to suppose that the land of those early days 
could not have been thrown up into lofty mountains ; for, if so, such great 
elevations must have been accompanied by corresponding deep chasms in 
the crust of the earth, and these would necessarily have been impassable 
barriers to the groups of marine creatures which have been described 
as more or less coexistent over wider regions *. Profound abysses of the 
ocean are, it is well known, as complete barriers to the migration of Fishes 
and many other marine creatures, as lofty mountains are to inhabitants 
of the land. The discovery, therefore, of the vast profundity of the ocean 
midway between the Cape of Good Hope and Cape Horn, amounting (it 
is said, even allowing for some amount of error in the soundings) to 7000 
fathoms, or nearly double the height of the loftiest mountains, is, in- 
deed, the strongest possible illustration of the impassable nature of such 
subaqueous barriers. These existing hollows have resulted from many 
great oscillations of the surface, in comparatively recent geological periods, 
which have caused great variations in the ' marine provinces ' of ex- 
isting animals, and placed them in strong contrast with the uniformity 
of the ancient wide -spread faunas. 
Duly estimating the great value of the knowledge of marine life ac- 
quired and applied by a naturalist whose researches, coupled with those 
of Loven and other cotemporaries, have thrown new light on many phe- 
nomena previously unexplained, let us guard against the inference that, 
because such acquaintance with the natural operations of our own era is 
applicable to the last geological data, or those of Tertiary age, it should also 
apply to the quasi-universal, and therefore very different, physical condi- 
tions under which primeval creatures existed. During the Tertiary period, 
the crust of the earth had, as we know, approximated considerably to its 
present varied outline ; and before it drew to a close, great changes had 
taken place, by which regions formerly occupied by animals and plants 
requiring a warm and equable climate were covered even by ice and gla- 
ciers. Whilst, therefore, we thank Edward Forbes for dredging the sea- 
bottoms, and teaching us that deep-sea mollusks are now living near high 
lands in the Mediterranean, whence pebbles may be so washed down as to 
lie in juxtaposition with the tenants of the deep, we must believe that 
either this argument cannot bear upon the primeval era, in which we have 
* Cuvier and Valenciennes showed (Hist. Poiss.) J. Eicharsdon (1836, 1845J lead us to infer that 
that very few species of Fish cross the Atlantic ; deep seas are as impassable barriers to Fishes as 
and the Keports to the British Association by Sir high mountains are to Mammals. 
