But with the fresh upheaval
and climatic disturbance at the end of the Jurassic, and during
the Cretaceous, they spread northward, and replaced the dying
reptiles, as the Angiosperms replaced the dying cycads. When they
met the spread of the Angiosperm vegetation they would receive
another great stimulus to development.
They appear in Europe and North America in the earliest
Cretaceous. The rise of the land had connected many hitherto
isolated regions, and they seem to have poured over every bridge
into all parts of the four continents. The obscurity of their
origin is richly compensated by their intense evolutionary
interest from the moment they enter the geological record. We
have seen this in the case of every important group of plants and
animals, and can easily understand it. The ancestral group was
small and local; the descendants are widely spread. While,
therefore, we discover remains of the later phases of development
in our casual cuttings and quarries, the ancestral tomb may
remain for ages in some unexplored province of the geological
world. If this region is, as we suspect, in Africa, our failure
to discover it as yet is all the more intelligible.
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