Now, this permanent retention of embryonic
characters and this "comprehensiveness" of structural type are
signs of what a zoologist considers to be a comparatively low
grade of organisation; and the prevalence of these features in
the earlier forms of animals is a very striking phenomenon, though
they are none the less perfectly organised so far as their own
type is concerned. As we pass upwards in the geological scale,
we find that these features gradually disappear, higher and ever
higher forms are introduced, and "specialisation" of type takes
the place of the former comprehensiveness. We shall have occasion
to notice many of the facts on which these views are based at
a later period, and in connection with actual examples. In the
meanwhile, it is sufficient to state, as a widely-accepted
generalisation of palaeontology, that there has been in the past
a general progression of organic types, and that the appearance
of the lower forms of life has in the main preceded that of the
higher forms in point of time.
PART II
HISTORICAL PALAEONTOLOGY
CHAPTER VII.
THE LAURENTIAN AND HURONIAN PERIODS.
The _Laurentian Rocks_ constitute the base of the entire stratified
series, and are, therefore, the oldest sediments of which we have
as yet any knowledge. They are more largely and more typically
developed in North America, and especially in Canada, than in
any known part of the world, and they derive their title from
the range of hills which the old French geographers named the
"Laurentides.
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