312 PROGRESS OF ORGANIZATION. 
genera, and even whole families, have been 
called into existence, and again totally annihi- 
lated, at various and successive periods, during 
the progress of the construction of the crust of 
our globe. 
The history of Chambered Shells tends fur- 
ther to throw light upon a point of importance 
in physiology, and shows that it is not always 
by a regular gradation from lower to higher 
degrees of organization, that the progress of life 
has advanced, during the early epochs of which 
geology takes cognizance. We find that many 
of the more simple forms have maintained 
their primeval simplicity, through all the varied 
changes the surface of the earth has undergone ; 
whilst, in other cases, organizations of a higher 
order preceded many of the lower forms of 
animal life ; some of the latter appearing, for the 
first time, after the total annihilation of many 
species and genera of a more complex cha- 
racter.* 
* The introduction, in the Tertiary periods, of a class of 
animals of lower organization, viz. the carnivorous Trachelipods, 
(See Chap. XV. Section 1,) to fill the place which, during the 
Secondary periods, had been occupied by a higher order, namely, 
the carnivorous Cephalopods, affords an example of Retrocession 
which seems fatal to that doctrine of regular Progression, which 
is most insisted on by those who are unwilling to admit the re- 
peated interferences of Creative power, in adjusting the successive 
changes that animal life has undergone. 
It will appear, on examination of the shells of fossil Nautili, 
that they have retained, through strata of all ages, their abori- 
