kinds found now in tropical waters; there are none like our Cods,
Haddocks, etc., such as are found at present in the colder seas. The
Fishes of the Jurassic epoch corresponding to our Sharks and Skates and
Gar-Pikes still exist, but in much smaller proportion, while these more
modern kinds are very numerous. Indeed, a classification of the
Cretaceous Fishes would correspond very nearly to one founded on those
now living. Shall we, then, suppose that the large reptilian Fishes of
the Jurassic time began suddenly to lay numerous broods of these
smaller, more modern, scaly Fishes? And shall we account for the
diminution of the previous forms by supposing that in order to give a
fair chance to the new kinds they brought them forth in large numbers,
while they reproduced their own kind less abundantly? According to very
careful estimates, if we accept this view, the progeny of the Jurassic
Fishes must have borne a proportion of about ninety per cent, of
entirely new types to some ten per cent, of those resembling the
parents. One would like a fact or two on which to rest so very
extraordinary a reversal of all known physiological laws of
reproduction, but, unhappily, there is not one.

Still more unaccountable, upon any theory of development according to
ordinary laws of reproduction, are those unique, isolated types limited
to a single epoch, or sometimes even to a single period. There are some
very remarkable instances of this in the Cretaceous deposits. To make my
statement clearer, I will say a word of the sequence of these deposits
and their division into periods.

These Cretaceous beds were at first divided only into three sets, called
the Neocomian, or lower deposits, the Green-Sands, or middle deposits,
and the Chalk, or upper deposits. The Neocomian, the lower division, was
afterwards subdivided into three sets of beds, called the Lower, Middle,
and Upper Neocomian by some geologists, the Valengian, Neocomian, and
Urgonian by others. These three periods are not only traced in immediate
succession, one above another, in the transverse cut before described,
across the mountain of Chaumont, near Neufchatel, but they are also
traced almost on one level along the plain at the foot of the Jura. It
is evident that by some disturbance of the surface the eastern end of
the range was raised slightly, lifting the lower or Valengian deposits
out of the water, so that they remain uncovered, and the next set of
deposits, the Ne