ial. We are entirely unable to explain on what physiological or
other laws this singular diversity in the habitations of living mammalia
depends; but nothing is more clear than that the causes which stamp so
peculiar a character on two different provinces of wide extent are
wholly independent of time, or of the age or maturity of the planet.

The strata of the Wealden, although of a later date than the oolite of
Stonesfield, and although filled with the remains of large reptiles,
both terrestrial and aquatic, have not yielded as yet a single marsupial
bone. Were we to assume on such scanty data that no warm-blooded
quadrupeds were then to be found throughout the northern hemisphere,
there would still remain a curious subject of speculation, whether the
entire suppression of one important class of vertebrata, such as the
mammiferous, and the great development of another, such as the
reptilian, implies a departure from fixed and uniform rules governing
the fluctuations of the animal world; such rules, for example, as appear
from one century to another to determine the growth of certain tribes of
plants and animals in arctic, and of other tribes in tropical regions.

In Australia, New Zealand, and many other parts of the southern
hemisphere, where the indigenous land quadrupeds are comparatively few,
and of small dimensions, the reptiles do not predominate in number or
size. The deposits formed at the mouth of an Australian river, within
the tropics, might contain the bones of only a few small marsupial
animals, which, like those of Stonesfield, might hereafter be discovered
with difficulty by geologists; but there would, at the same time, be no
megalosauri and other fossil remains, showing that large saurians were
plentiful on the land and in the waters at a time when mammalia were
scarce. This example, therefore, would afford a very imperfect parallel
to the state of the animal kingdom, supposed to have prevailed during
the secondary periods, when a high temperature pervaded European
latitudes.

It may nevertheless be advantageous to point to some existing anomalies
in the geographical development of distinct classes of vertebrata which
may be comparable to former conditions of the animal creation brought to
light by geology. Thus in the arctic regions, at present, reptiles are
small, and sometimes wholly wanting, where birds, large land quadrupeds,
and cetacea abound. We meet with bears, wolves, foxes, musk oxen, and