170 
THE GEOLOGICAL MIDDLE AGE. 
our common Fishes, none corresponding to our 
Herring, Pickerel, Mackerel, and the like, — no 
Fishes, in short, with thin membranous scales, 
but that the class was represented exclusively by 
those with hard, flint-like scales. In the Creta- 
ceous epoch, however, we come suddenly upon a 
horde of Fishes corresponding to our smaller 
common Fishes of the Pickerel and Herring 
tribes, but principally of the 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 numer- 
ous. 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 or- 
der to give a fan chance to the new kinds they 
brought them forth in large numbers, while they 
reproduced their own kind less abundantly ? Ac- 
cording to very careful estimates, if we accept 
this view, the progeny of the Jurassic Fishes 
must have borne a proportion of about ninety per 
