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KIRTLEY F. MATHER 
brachiopod shells which weather out of rocks dating back to 
the very oldest of fossiliferous strata are scarcely distinguishable 
from the shells of living brachiopods washed ashore by the waves 
which are daily exhuming the relics of antiquity from the cliffs 
against which they dash into spray. 
Again, these believers in the continuity of life, regardless 
of local disturbances, adversities and cataclysms, called atten- 
tion to the fact that the fossil remains of successive groups of 
plants or animals were as a rule indicative of creatures only 
slightly modified from those which had preceded them. Abrupt 
and sweeping changes in the assemblage of living beings were 
apparent whenever there were gaps in the record of life, but when 
the missing chapters were supplied by more extensive study the 
vital stream Was shown to have been flowing on its way un- 
broken. Proof was not lacking that the presence of an altogether 
different assemblage of fossils in two successive series of rocks 
was due to the migration into that locality of creatures who had 
been slowly developing elsewhere. The pulse of life might beat 
a little irregularly now and then, but never had it been entirely 
suspended. 
Gradually this notion of continuity of life, regardless of seem- 
ing interruptions in the record, gained sway, and at the present 
time there are no paleontologists who entertain any other idea 
of organic development. But still our question is unanswered. 
Granting the unbroken flow of the stream of life, what part in 
the progress of that stream has been played by the rapids and 
cataracts? Have they been essential to its onward flow, or 
are they only spectacular displays, actually of less importance 
than the quiet reaches where the deeper waters move? 
Ill 
The close of the Paleozoic Era — that long interval of ancient 
time during which invertebrates and fishes, with later the 
addition of amphibians and reptiles, were the only animals upon 
the earth — was marked by a series of adverse episodes so dis- 
turbing in their influence upon plants and animals that it brought 
about what is commonly called a ^T’evolution’^ in the organic 
