he commencement
of the chalk formation, the fact would certainly be highly remarkable;
but I cannot see that it would be an insuperable difficulty on my
theory, unless it could likewise be shown that the species of this group
appeared suddenly and simultaneously throughout the world at this same
period. It is almost superfluous to remark that hardly any fossil-fish
are known from south of the equator; and by running through Pictet's
Palaeontology it will be seen that very few species are known from
several formations in Europe. Some few families of fish now have a
confined range; the teleostean fish might formerly have had a similarly
confined range, and after having been largely developed in some one sea,
might have spread widely. Nor have we any right to suppose that the seas
of the world have always been so freely open from south to north as
they are at present. Even at this day, if the Malay Archipelago were
converted into land, the tropical parts of the Indian Ocean would form
a large and perfectly enclosed basin, in which any great group of marine
animals might be multiplied; and here they would remain confined, until
some of the species became adapted to a cooler climate, and were enabled
to double the southern capes of Africa or Australia, and thus reach
other and distant seas.

From these and similar considerations, but chiefly 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 ideas
on many points, which the discoveries of even the last dozen years have
effected, it seems to me to be about as rash in us to dogmatize on the
succession of organic beings throughout the world, as it would be for
a naturalist to land for five minutes on some one 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 graver. I allude
to the manner in which numbers of species of the same group, 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
have descended from one progenitor, apply with nearly equal force to
the earliest known species. For instance, I cannot doubt that all the
Silurian trilobites have descended from some one crustacean, which must