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Pedigrees of Plants
We have here, therefore, a most interesting evolutionary group.
The botanist finds even more difficulty than the zoologist in
drawing up the pedigrees of his plants, but the general features
of the larger groups which he finds in succession in the
chronicle of the earth point very decisively to evolution. The
seed-bearing ferns of the Coal-forest point upward to the later
stage, and downward to a common origin with the ordinary
spore-bearing ferns. Some of them are "altogether of a cycadean
type" (Scott) in respect of the seed. On the other hand, the
Bennettiteae of the Jurassic have the mixed characters of ferns,
cycads, and flowering plants, and thus, in their turn, point
downward to a lower ancestry and upward to the next great stage
in plant-development. It is not suggested that the seed-ferns we
know evolved into the cycads we know, and these in turn into our
flowering plants. It is enough for the student of evolution to
see in them so many stages in the evolution of plants up to the
Angiosperm level. The gaps between the various groups are less
rigid than scientific men used to think.
Taller than the cycads, firmer in the structure of the wood, and
destined to survive in thousands of species when the cycads would
be reduced to a hundred, were the pines and yews and other
conifers of the Jurassic landscape. We saw them first appearing,
in the stunted Walchias and Voltzias, during the severe
conditions of the Permian period. Like the birds and mammals they
await the coming of a fresh period of cold to give them a decided
superiority over the cycads. Botanists look for their ancestors
in some form related to the Cordaites of the Coal-forest. The
ginkgo trees seem to be even more closely related to the
Cordaites, and evolved from an early and generalised branch of
that group. The Cordaites, we may recall, more or less united in
one tree the characters of the conifer (in their wood) and the
cycad (in their fruit).
So much for the evolutionary aspect of the Jurassic vegetation in
itself. Slender as the connecting links are, it points clearly
enough to a selection of higher types during the Permian
revolution from the varied mass of the Carboniferous flora, and
it offers in turn a singularly varied and rich group from which a
fresh selection may choose yet higher types. We turn now to
consider the animal population which, directly or indirectly, fed
upon it, and grew with its growth. To the reptiles, the birds,
and the mammals, we must devote special chapters. Here we may
briefly survey the less conspicuous animals of the Mesozoic
Epoch.
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