tant seas.

From these considerations, from our ignorance of the geology of other
countries beyond the confines of Europe and the United States, and
from the revolution in our palaeontological knowledge effected by the
discoveries of the last dozen years, it seems to me to be about as rash
to dogmatize on the succession of organic forms throughout the world, as
it would be for a naturalist to land for five minutes on a barren
point in Australia, and then to discuss the number and range of its
productions.

ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN THE LOWEST KNOWN
FOSSILIFEROUS STRATA.

There is another and allied difficulty, which is much more serious. I
allude to the manner in which species belonging to several of the main
divisions of the animal kingdom suddenly appear in the lowest known
fossiliferous rocks. Most of the arguments which have convinced me that
all the existing species of the same group are descended from a single
progenitor, apply with equal force to the earliest known species.
For instance, it cannot be doubted that all the Cambrian and Silurian
trilobites are descended from some one crustacean, which must have lived
long before the Cambrian age, and which probably differed greatly from
any known animal. Some of the most ancient animals, as the Nautilus,
Lingula, etc., do not differ much from living species; and it cannot on
our theory be supposed, that these old species were the progenitors of
all the species belonging to the same groups which have subsequently
appeared, for they are not in any degree intermediate in character.

Consequently, if the theory be true, it is indisputable that before the
lowest Cambrian stratum was deposited long periods elapsed, as long as,
or probably far longer than, the whole interval from the Cambrian age
to the present day; and that during these vast periods the world swarmed
with living creatures. Here we encounter a formidable objection; for it
seems doubtful whether the earth, in a fit state for the habitation of
living creatures, has lasted long enough. Sir W. Thompson concludes that
the consolidation of the crust can hardly have occurred less than twenty
or more than four hundred million years ago, but probably not less than
ninety-eight or more than two hundred million years. These very wide
limits show how doubtful the data are; and other elements may have
hereafter to be introduced into the problem. Mr. Croll estimates that
about sixty mil