e--assuming always, for reasons before stated, that single
stocks only of each animal and plant are originally created, and that
individuals of new species did not suddenly start up in many different
places at once.

"So imperfect has the science of natural history remained down to our
own times that, within the memory of persons now living, the numbers
of known animals and plants have doubled, or even quadrupled, in many
classes. New and often conspicuous species are annually discovered in
parts of the old continent long inhabited by the most civilized nations.
Conscious, therefore, of the limited extent of our information, we
always infer, when such discoveries are made, that the beings in
question bad previously eluded our research, or had at least existed
elsewhere, and only migrated at a recent period into the territories
where we now find them.

"What kind of proofs, therefore, could we reasonably expect to find of
the origin at a particular period of a new species?

"Perhaps, it may be said in reply, that within the last two or three
centuries some forest tree or new quadruped might have been observed to
appear suddenly in those parts of England or France which had been most
thoroughly investigated--that naturalists might have been able to show
that no such being inhabited any other region of the globe, and that
there was no tradition of anything similar having been observed in the
district where it had made its appearance.

"Now, although this objection may seem plausible, yet its force will be
found to depend entirely on the rate of fluctuation which we suppose
to prevail in the animal world, and on the proportions which such
conspicuous subjects of the animal and vegetable kingdoms bear to those
which are less known and escape our observation. There are perhaps
more than a million species of plants and animals, exclusive of the
microscopic and infusory animalcules, now inhabiting the terraqueous
globe, so that if only one of these were to become extinct annually, and
one new one were to be every year called into being, much more than a
million of years might be required to bring about a complete revolution
of organic life.

"I am not hazarding at present any hypothesis as to the probable rate
of change, but none will deny that when the annual birth and the annual
death of one species on the globe is proposed as a mere speculation,
this, at least, is to imagine no slight degree of instability in the
anima