For instance, the
Temperate Zone throughout the world is characterized by certain families
of trees: by Oaks, Maples, Beeches, Birches, Pines, etc.; but the Oaks,
Maples, Beeches, Birches, and the like, of the American flora in that
latitude differ in species from the corresponding European flora. So
in the Carboniferous period, when more uniform climatic conditions
prevailed throughout the world, the character of the vegetation showed a
general unity of structure everywhere; but it was nevertheless broken up
into distinct botanical provinces by specific differences of the same
kind as those which now give such diversity of appearance to the
vegetation of the Temperate Zone in Europe as compared with that of
America, or to the forests of South America as compared with those of
Africa.
There can be no doubt as to the true nature of the Carboniferous
forests; for the structural character of the trees is as strongly marked
in their fossil remains as in any living plants of the same character.
We distinguish the Ferns not only by the peculiar form of their leaves,
often perfectly preserved, but also by the fructification on the lower
surface of the leaves, and by the distinct marks made on the stem at
their point of juncture with it. The leaf of the Fern, when falling,
leaves a scar on the stem varying in shape and size according to the
kind of Fern, so that the botanist readily distinguishes any particular
species of Fern by this means,--a birth-mark, as it were, by which he
detects the parentage of the individual.
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