, gave rise to this very remarkable form in the class,
allowed it to die out, and revived it again at various intervals? This
is by no means the only instance of the same kind. There are a number of
types making their appearance suddenly, lasting during one period or
during a succession of periods, and then disappearing forever, while
others, like the Rudistes, come in, vanish, and reappear at a later
time.

[Illustration: Rudistes.]

I am well aware that the advocates of the development-theory do not
state their views as I have here presented them. On the contrary, they
protest against any idea of sudden, violent, abrupt changes, and
maintain that by slow and imperceptible modifications during immense
periods of time these new types have been introduced without involving
any infringement of the ordinary processes of development; and they
account for the entire absence of corroborative facts in the past
history of animals by what they call the "imperfection of the geological
record." Now, while I admit that our knowledge of geology is still very
incomplete, I assert that just where the direct sequence of geological
deposits is needed for this evidence, we have it. The Jurassic beds,
without a single modern scaly Fish, are in immediate contact with the
Cretaceous beds, in which the Fishes of that kind are proportionately
almost as numerous as they are now; and between these two sets of
deposits there is not a trace of any transition or intermediate form to
unite the reptilian Fishes of the Jurassic with the common Fishes of the
Cretaceous times. Again, the Cretaceous beds in which the crowded banks
of Rudistes, so singular and unique in form, first make their
appearance, follow immediately upon those in which all the Bivalves are
of an entirely different character. In short, the deposits of this year
along any sea-coast or at the mouth of any of our rivers do not follow
more directly upon those of last year than do these successive sets of
beds of past ages follow upon each other. In making these statements, I
do not forget the immense length of the geological periods; on the
contrary, I fully accede to it, and believe that it is more likely to
have been underrated than overstated. But let it be increased a
thousand-fold, the fact remains, that these new types occur commonly at
the dividing line where one period joins the next, just on the margin of
both.

For years I have collected daily among some of these deposits,