174 
THE GEOLOGICAL MIDDLE AGE. 
means rare. They are found always in great 
numbers, occupying crowded beds, like Oysters 
in the present time. So numerous are they, where 
they occur at all, that the deposits containing 
them are called by many naturalists the first, sec- 
ond, third, and fourth hank of Rudistes. Which 
of the ordinary Bivalves, then, 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 mak- 
ing 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. 
I am well aware that the advocates of the de- 
velopment-theory do not state their views as I 
have here presented them. On the contrary, they 
protest against any idea of sudden, violent, ab- 
rupt changes, and maintain that by slow and im- 
perceptible modifications during immense periods 
of time these new types have been introduced 
without involving any infringement of the ordi- 
nary 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 
