ter enclosing the seeds, and, as it is an advantage to the
plant that its seeds be scattered beyond the already populated area, by
passing through the alimentary canal of the bird, and being discharged
with its excrements, a fresh line of evolution leads to the appearance
of the large and coloured fruits. The birds, again, turn upon the
swarming insects, and the steady selection they exercise leads to
the zigzag flight and the protective colour of the butterfly, the
concealment of the grub and the pupa, the marking of the caterpillar,
and so on. We can understand the living nature of to-day as the outcome
of that teeming, striving, changing world of the Tertiary Era, just as
it in turn was the natural outcome of the ages that had gone before.

CHAPTER XVII. THE ORIGIN OF OUR MAMMALS

In our study of the evolution of the plant, the insect, and the bird we
were seriously thwarted by the circumstance that their frames,
somewhat frail in themselves, were rarely likely to be entombed in good
conditions for preservation. Earlier critics of evolution used, when
they were imperfectly acquainted with the conditions of fossilisation,
to insinuate that this fragmentary nature of the geological record was a
very convenient refuge for the evolutionist who was pressed for positive
evidence. 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 t