9 
with the highest of organized beings, and thus requires in 
common with them certain conditions necessary for its 
existence ; but although thus highly placed in the scale of 
beings, it is at the same time by the peculiarity of its 
respiration, (fishes being water-breathing animals,) linked to 
the lowest grades of the animal kingdom, with which until 
the close of the carboniferous era it had been doomed solely 
to associate. I repeat, then, that the class of fishes, from 
its peculiar position in the zoological scale, from its consist- 
ing of vertebrated animals, but still water-breathing ani- 
mals, is calculated as much, and probably more, than any 
other class to aid the inquiries of the geologist, to throw 
light upon the physical condition of this earth and its 
atmosphere during the time the carboniferous strata were 
deposited. And it cannot be regarded but as a subject of 
profound interest to ascertain what were the physical 
conditions of the surface of this planet which, at the period 
alluded to, allowed of the existence of vertebrated animals, 
but of such only as respired through the medium of water ; 
and again what physical changes had been effected at 
the termination of the carboniferous era, so as to allow of 
the existence of air-breathing reptiles which in the varied 
forms of saurians crowded the ocean during the deposition 
of the Lias and subsequent formations. I cannot resist the 
temptation of here briefly noticing the conjectures of M. 
Brongniart on this interesting subject. Conjectures indeed 
they only are, but where all is darkness, or light but 
dimly seen, conjecture may be the herald of discovery. 
Considering how prodigiously luxuriant must have been the 
vegetation in the carboniferous era, in order to have pro- 
duced those enormous masses of vegetable matter \^\ich 
form the coal strata, and considering that tliis excessively 
luxuriant vegetation could not have been supported by a 
soil highly nutritious, from being strongly impregnated with 
carbon, like our own soil, from the wreck of pre-existing 
vegetation, M. Brongniart is led to inquire, from what 
source could these plants derive the carbon necessary for 
their enormous and rapid growth ? Tlie only source that 
conjecture can supply is the atmosphere, and M. Brongniart 
supposes that, during the carboniferous era, and previous to 
that period, the atmosphere of this planet differed essen- 
tially from its present condition, in being, not, as now, only 
slightly charged with carbonic acid, but intensely impreg- 
nated with this gas ; and as we know that in our own 
atmosphere plants absorb carbonic acid and appropriate its 
carbon to their nutrition, so in these ancient periods of the 
globe the vegetation may have derived its support from this 
source, and to the great fertility of this source may not 
improbably be due the rich luxuriance of the carboniferous 
vegetation. And surely this conjecture gains no little force 
