| | | | | |Plane. Singly involute.
| | | | | | | |Hemispheres small;
| | | | | | | |mesencephalon sometimes exposed
| | | | | | | |
Lower (Puerco.)
|5-5
| |Plantigrade.
| | |Flat.
| | | |Opposite.
| | | | |Smooth.
| | | | | |3-tubercles. (4-tubercles), none crested.
| | | | | | |Plane.
| | | | | | | |Mesencephalon exposed;
| | | | | | | |hemisphere small and smoother.
| | | | | | | |

The evidence, then, which is furnished by all parts of the vertebral
skeleton--whether we have regard to Fishes, Reptiles, Birds, or
Mammals--is cumulative and consistent. Nowhere do we meet with any
deviation or ambiguity, while everywhere we encounter similar proofs of
continuous transformation--proofs which vary only with the varying
amount of material which happens to be at our disposal, being most
numerous and detailed in those cases where the greatest number of fossil
forms has been preserved by the geological record. Here, therefore, we
may leave the vertebral skeleton; and, having presented a sample of the
evidence as yielded by horns and bones, I will conclude by glancing with
similar brevity at the case of shells--which, as before remarked,
constitute the only other sufficiently hard or permanent material to
yield unbroken evidence touching the fossil ancestry of animals.

Of course it will be understood that I am everywhere giving merely
samples of the now superabundant evidence which is yielded by
palaeontology; and, as this chapter is already a long one, I must content
myself with citing only the case of mollusk-shells, although shells of
other classes might be made to yield highly important additions to the
testimony. Moreover, even as regards the one division of mollusk-shells,
I can afford to quote only a very few cases. These, however, are in my
opinion the strongest single pieces of evidence in favour of
transmutation which have thus far been brought to light.

Near the village of Steinheim, in Wuertemberg, there is an ancient
lake-basin, dating from Tertiary times. The lake has long ago dried up;
but its aqueous deposits are extraordinarily rich in fossil shells,
especially of different species of the genus _Planorbis_. The following