eriod proves, there was a far more
extensive sameness than we see at present, simply because the
distribution of heat was more uniform and climates were less marked. But
from this point the diversity of form in climate distribution becomes
more and more conspicuous, though we must descend, perhaps, as late as
the Wealden before we discover any flowering plants, except Gymnosperms,
as Conifers and Cycads. All this is what might be expected on the
doctrine of external influence, but not on the doctrine of an innate and
interior developmental force.

If, at this stage, attention is once again turned to the animal
kingdom, we find our opinion confirmed. The diminution of carbonic
acid in the atmosphere, the deposit of coal in the earth, the
precipitation of carbonate of lime in the sea, the disengagement of an
increased quantity of oxygen in the air, and the reduction of
atmospheric pressure--different effects contemporaneously
occurring--were soon followed by the consequence which they made
possible--the appearance of hot-blooded mammals. [Sidenote: Cold and
hot-blooded animals.] Perhaps those first arising might, like our
hibernates, lead a sluggish existence, with imperfect respiration;
but, as the media improved and the temperature declined, more vigorous
forms of life emerged, though we have probably to descend to the
Tertiary epoch before we meet with birds, which of all animals have
the most energetic respiration, and possess the highest heat.

[Sidenote: The organisms of the sea.] As with the atmosphere, so with
the sea. Variations in its composition must control the organisms it
contains. With its saline constituents its life must change. Before the
sunlight had removed from the atmosphere so much of its carbonic acid,
decomposing it through the agency of plants, the weight of carbonate of
lime held in solution by the highly carbonated water was far greater
than was subsequently possible, and the occurrence of limestone became a
necessary event. With such a disturbance in the composition of the
sea-water, its inhabiting organisms were necessarily disturbed. And so
again, subsequently, when the solar heat began to preponderate on the
surface over the subsiding interior heat, the constitution of the
sea-water, as respects its salinity, was altered through difference of
evaporation in different latitudes, an effect inevitably making a
profound impression on marine animal life.

[Sidenote: Nature of hereditary trans