The
complaint is no longer found in any serious work. Where we find
excellent conditions for preservation, and animals suitable for
preservation living in the midst of them, the record is quite
satisfactory. We saw how the chalk has yielded remains of
sea-urchins in the actual and gradual process of evolution.
Tertiary beds which represent the muddy bottoms of tranquil lakes
are sometimes equally instructive in their fossils, especially of
shell-fish. The Paludina of a certain Slavonian lake-deposit is a
classical example. It changes so greatly in the successive levels
of the deposit that, if the intermediate forms were not
preserved, we should divide it into several different species.
The Planorbis is another well-known example. In this case we have
a species evolving along several distinct lines into forms which
differ remarkably from each other.
The Tertiary mammals, living generally on the land and only
coming by accident into deposits suitable for preservation,
cannot be expected to reveal anything like this sensible advance
from form to form. They were, however, so numerous in the
mid-Tertiary, and their bones are so well calculated to survive
when they do fall into suitable conditions, that we can follow
their development much more easily than that of the birds.
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