176 
THE GEOLOGICAL MIDDLE AGE. 
of those deposits, and I know the Sea-Urchins, 
Corals, Fishes, Crustacea, and Shells of those old 
shores as well as I know those of Nahant Beach, 
and there is nothing more striking to a natural- 
ist than the sudden, abrupt changes of species in 
passing from one to another. In the second set 
of Cretaceous beds, the Neocomian, there is found 
a little Terebratula (a small Bivalve Shell) in 
immense quantities: they may actually be col- 
lected by the bushel. Pass to the Urgonian beds, 
resting directly upon the Neocomian, and there is 
not one to be found, and an entirely new species 
comes in. There is a peculiar Spatangus (Sea- 
Urchin) found throughout the whole series of 
beds in which this Terebratula occurs. At the 
same moment that you miss the Shell, the Sea- 
Urchin disappears also, and another takes its 
place. Now, admitting for a moment that the 
later can have grown out of the earlier forms, I 
maintain, that, if this be so, the change is imme- 
diate, sudden, without any gradual transitions, 
and is, therefore, wholly inconsistent with all our 
known physiological laws, as well as with the 
transmutation-theory. 
There is a very singular group of Ammonites 
in the Cretaceous epoch, which, were it not for 
the suddenness of its appearance, might seem 
rather to favor the development-theory, from its 
great variety of closely allied forms. We have 
