230 THE OLD RED SANDSTONE. 
could have been based, as in the supposed case, on a period 
perhaps a hundred times more extended ? 
There exist wonderful analogies in nature between the 
geological history of the vertebrated animals as an order, and 
the individual history of every mammifer — between the his- 
tory, too, of fish as a class, and that of every single fish. 
“It has been found by Tiedemann,” says Mr. Lyeli, * that 
the brain of the foetus in the higher class of vertebrated ani- 
mals assumes in succession the various forms which belong 
to fishes, reptiles, and birds, before it acquires those additions 
and modifications which are peculiar to the mammiferous 
tribes.” ‘In examining the brain of the mammalia,” says M. 
Serres, “at an early stage of life, you perceive the cerebral 
hemispheres consolidated, as in fish, in two vesicles isolated 
one from the other; at a later period you see them affect 
the configuration of the cerebral hemispheres of reptiles ; 
still later, again, they present you with the forms of those of 
birds; and finally, at the era of birth, the permanent forms 
which the adult mammalia present.” And such seems to 
have been the history of the vertebrata as an order, as cer- 
tainly as that of the individual mammifer. The fish preceded 
the reptile in the order of creation, just as the crustacean had 
preceded the fish, and the annelid the crustacean. Again, 
though the fact be somewhat more obscure, the reptile seems 
to have preceded the bird. We find, however, unequivocal 
traces of the feathered tribes in well-marked foot-prints im- 
pressed on a sandstone in North America, at most not more 
modern than the Lias, but which is generally supposed to be 
of the same age with the New Red Sandstone of Germany 
and our own country. In the Oolite — at least one, perhaps 
two formations later — the bones of the two species of mammif- 
erous quadrupeds have been found, apparently of the marsu- 
