" It is a matter of
much importance to geology to compare the present distribution of plants
over the earth's surface with that exhibited in the fossil floras of the
primitive world. The temperate zone of the southern hemisphere, which is so
rich in seas and islands, and where
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tropical forms blend so remarkably with those of colder parts of the earth,
presents according to Darwin's beautiful and animated descriptions,* the
most instructive materials for the study of the present and the past
geography of plants.
[footnote] *Charles Darwin, 'Journal of the Voyages of the Adventure and
Beagle', 1839, p. 271.
The history of the primordial ages is, in the strict sense of the word, a
part of the history of plants.
Cycadeae, which, from the number of their fossil species, must have occupied
a far more important part in the extinct than in the present vegetable
world, are associated with the nearly allied Coniferae from the coal
formations upward. They are almost wholly absent in the epoch of the
variegated sandstone which contains Coniferae of rare and luxuriant
structure ('Voltizia, Haidingera, Albertia'); the Cycadeae, however, occur
most frequently in the keuper and lias strata, in which more than twenty
different forms appear.
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