edition, page 90, the following uncompromising statement of immutability:--

"The meaning attached to the term species, in natural history, is very
simple and obvious. It includes only one circumstance--namely, an original
distinctness and constant transmission of any character. A race of
animals, or plants, marked by any peculiarities of structure which have
always been constant and undeviating, constitutes a species."

On page 91, in speaking of the idea that the species which make up a genus
may have descended from a common form, he says:--

"There must, indeed, be some principle on which the phenomena of
resemblance, as well as those of diversity, may be explained; and the
reference of several forms to a common type seems calculated to suggest the
idea of some original affinity; but, as this is merely a conjecture, it
must be kept out of sight when our inquiries respect matters of fact only."

This view is again given in Volume II., page 569, where he asks whether we
should believe that "at the first production of a genus, when it first grew
into existence, some slight modification in the productive causes stamped
it originally with all these specific diversities? Or is it most probable
that the modification was subsequent to its origin, and that the genus at
its first creation was one and uniform, and afterwards became diversified
by the influence of external agents?" He concludes that "the former of
these suppositions is the conclusion to which we are led by all that can be
ascertained respecting the limits of species, and the extent of variation
under the influence of causes at present existing and operating."

In spite of the fact that Prichard did not carry his ideas to their logical
conclusion, it may perhaps excite surprise that Mr. Darwin should have