273 
THE CHANGES WHICH OUR PLANET HAS UNDERGONE. 
those now existing ; though he has not found and the oolite rocks, it is a highly interesting 
more than one species exactly the same. fact, that the capsule of the eye has been pre- 
Those of the formation called “ crag,” in served: and in many species from Monte 
Norfolk, are allied to the species now inha- Bolca, Solenhofen, and the lias, we see dis- 
biting the tropical seas. In the London clay, tinctly all the little blades which form the 
the beds in the basin of Paris, and at Monte branchiae. 
Bolca, about two-thirds belong to existing 
genera. 
In the formations next below these, the 
chalk, about one third only belong to 
existing genera. 
In the formations older than the chalk, 
there is not a single genus identical with the 
recent. The oolitic series, to the lias inclusive, 
forms by its species of fossil fish a very na- 
tural and well-defined group. The weald 
formation is included in this, in which M. 
Agassiz did not find a singlespecies referrible 
even to the genera of the chalk. 
Throughout the series of rocks deposited 
in these epochs, the two orders which pre^ 
vail by so large a majority (as above stated) 
in the existing creation, are not to be found. 
New species have since been created ; the 
whole genera formed of all those species are 
new, not merely in a few instances, but 
through such a range and extent, that even 
the entire order comprising those genera is 
new. In these older strata, on the other 
hand, different species, genera, and orders, 
existed in proportional abundance, most of 
which have since died away and disappeared ; 
and the two great orders, which at the present 
day form a small minority, were then pre- 
dominant. 
The most striking characteristics, per- 
haps, of^these periods are the predominance 
of those Ganoidians which have a symmetri- 
cal caudal-fin ; and those Placoi'dians, which 
have their teeth furrowed on both sides, and 
have large thorny rays on the dorsal-fin. 
These fossil rays had long been known, but 
their real nature was wholly misunderstood. 
In the formations below the lias, the 
character, above stated, in the tail-fin of the 
Ganoidians is entirely changed. Instead of 
n tail parting off into two equal and similar 
divisions or lobes, the backbone is continued 
straight on, into a true tail, while another 
lobe, or fin, is formed beneath, so as to give 
the appearance of a tail-fin, with two un- 
equal, unsymmetrical, lobes. This distinc- 
tion prevails up to the fishes of the most an- 
cient strata. 
The form of the teeth is another important 
distinction, bearing obviously a direct rela- 
tion to the habits of the animal and its means 
of subsistence. 
In strata more recent than those contain- 
ing coal, we find no fish decidedly carnivo- 
rous, — that is, provided with large conical 
and pointed teeth. In these strata, up to the 
chalk, the fish appear to have been omnivo- 
rous, their teeth being either |rounded, or in 
obtuse cones, or like a brush. The nature of 
the food of these fish is also ascertained by 
the discovery of the fossil contents of their 
intestines, in which scales of other fish, on 
which they had preyed, have been found. 
In a great number of instances from the 
tertiary beds of the Isle of Sheppy, the chalk. 
In the strata below the lias, we begin to 
find the largest of those large fish, of an 
organization allied to the Saurian, or lizard 
tribe ; the resemblance is chiefly in the mode 
of connexion of certain parts of the skeleton, 
and the form of the teeth. 
Froiii the general distribution of the species 
of fossil fish thus investigated, M. Agassiz has 
deduced some important and profound in- 
ferences, with regard to the clianges which 
our planet has undergone at remote epochs. 
There is a remarkable distinction between 
these fossil fishes, and the fossil zoophytes 'md 
testacea. Of these last, the same genera are 
found through several different formations, as 
we have already noticed ; and their organi- 
zation was such as enabled them to live 
through all the great changes in the physical 
condition of the globe, which accompanied 
the successive depositions of those formations. 
With the fossil ^s/ies the case is widely differ- 
ent. We have seen that the several genera, 
and even orders, vary extremely from one 
formation to another. Thus the changes in 
the constitution of the globe, which accom- 
paiiied the successive epochs, were of such 
a kind as these genera of fishes were unabla 
to survive. We see at once, then, a reason 
for this difference between them and the infe- 
rior classes, in the greater perfection and deli- 
cacy of their organization ; their more complica- 
ted structure required important modifications, 
according as great changes took place in the’ 
climate, and various physical relations in the 
order of things on the surface of the globe. Here 
then was the same beautiful series of adapta- 
tions, existing in as high perfection myriads of 
ages ago as at the present time : displayed 
equally in all the long series of creations, by 
which the globe has been gradually brought 
into its present condition, and evincing the 
ever-enduring and universal influence of the 
same creative Power and Intelligence. 
The fishes of each of the great periods of the 
earth’s formation are thus essentially diffeient 
from each other, but each series agreeing 
among themselves in some peculiarities of 
organization. The multitude of species so 
coexisting must, doubtless, have been fitted by 
that peculiar organization for the particular 
conditions which prevailed on the surface of 
the globe at the time they lived. So, likewise, 
the disappearance of whole species and genera, 
is the evidence of great and universal changes 
in those attendant conditions of the external 
world, which introduced a new order of things 
unsuited to that peculiar organization with 
which they were furnished. Thus, not only 
individuals, but whole families and species 
perished ; not only a few species, but a wide 
range of species, comprising a whole genus; 
and not only this, but so many genera as made 
up the larger portion of an entire order. One 
common peculiarity constituted the distinc- 
tion of the order ; that peculiarity was no 
