is an authoritative _resume_ of the facts.

As the deposits seem to have been continuous for ages, and the
fossil shells very abundant, this seemed to be an excellent
opportunity to test the theory of derivation. With this end in
view, they have been made the subject of exhaustive study by
Hilgendorf in 1866, and by Hyatt in 1880. In passing from the
lowest to the highest strata the species change greatly and many
times, the extreme forms being so different that, were it not for
the intermediate forms, they would be called not only different
species, but different genera. And yet the gradations are so
insensible that the whole series is nothing less than a
demonstration, in this case at least, of origin of species by
derivation with modifications. The accompanying plate of successive
forms (Fig. 89), which we take from Prof. Hyatt's admirable memoir,
will show this better than any mere verbal explanation. It will be
observed that, commencing with four slight varieties--probably
sexually isolated varieties--of one species, each series shows a
gradual transformation as we go upward in the strata--i. e. onward
in time. Series I branches into three sub-series, in two of which
the change of form is extreme. Series IV is remarkable for great
increase in size as well as change in form. In the plate we give
only selected stages, but in the fuller plates of the memoir, and
still more in the shells themselves, the subtilest gradations are
found[19].

[19] Le Conte, _loc. cit._, pp. 236-7.

[Illustration: FIG. 89.--Transmutations of _Planorbis_. (After
Hyatt.)]

Here is another and more recently observed case of transmutation in the
case of mollusks.

The recent species, _Strombus accipitrinus_, still inhabits the coasts
of Florida. Its extinct prototype, _S. Leidy_, was discovered a few
years ago by Prof. Heilprin in the Pliocene formations of the interior
of Florida. The peculiar shape of the wing, and tuberculation of the
whorl, are thus proved to have grown but of a previously more conical
form of shell.

[Illustration: FIG. 90.--Transformation of _Strombus_. (After
Heilprin.) 1, 1_a_, _Strombus Leidy_ (1, typical), Pliocene; 2,
2_a_, _Strombus accipitrinus_ (2_a_ typical) Recent.]

Lastly, attention may here again be directed to the very instructive
series of shells which has al