geological evidence of transition than he finds, and that all the most
eminent paleontologists maintain the immutability of species.

The general fact, however, that the fossil fauna of each period as a whole
is nearly intermediate in character between the preceding and the
succeeding faunas, is much relied on. We are brought one step nearer to the
desired inference by the similar "fact, insisted on by all paleontologists,
that fossils from two consecutive formations are far more closely related
to each other than are the fossils of two remote formations. Pictet gives a
well-known instance--the general resemblance of the organic remains from the
several stages of the chalk formation, though the species are distinct at
each stage. This fact alone, from its generality, seems to have shaken
Prof. Pictet in his firm belief in the immutability of species" (p. 335).
What Mr. Darwin now particularly wants to complete his inferential evidence
is a proof that the same gradation may be traced in later periods, say in
the Tertiary, and between that period and the present; also that the later
gradations are finer, so as to leave it doubtful whether the succession is
one of species--believed on the one theory to be independent, on the other,
derivative--or of varieties, which are confessedly derivative. The proof of
the finer gradation appears to be forthcoming. Des Hayes and Lyell have
concluded that many of the middle Tertiary and a large proportion of the
later Tertiary mollusca are specifically identical with living species; and
this is still the almost universally prevalent view. But Mr. Agassiz states
that, "in every instance where he had sufficient materials, he had found
that the species of the two epochs supposed to be identical by Des Hayes
and Lyell were in reality distinct, although closely allied species."[I-11]
Moreover, he is now satisfied, as we understand, that the same gradation is
traceable not merely in each great division of the Tertiary, but in
particular deposits or successive beds, each answering to a great number of
years; where what have passed unquestioned as members of one species, upon