state, that the order of events thus assumed to occur, for
the sake of illustration, must be in harmony with all the conclusions
legitimately drawn by geologists from the structure of the earth, and
must be equally in accordance with the changes observed by man to be now
going on in the living as well as in the inorganic creation. It may be
necessary in the present state of science to supply some part of the
assumed course of nature hypothetically; but if so, this must be done
without any violation of probability, and always consistently with the
analogy of what is known both of the past and present economy of our
system. Although the discussion of so comprehensive a subject must carry
the beginner far beyond his depth, it will also, it is hoped, stimulate
his curiosity, and prepare him to read some elementary treatises on
geology with advantage, and teach him the bearing on that science of the
changes now in progress on the earth. At the same time it may enable him
the better to understand the intimate connection between the second and
third books of this work, the former of which is occupied with the
changes in the inorganic, the latter with those of the organic creation.

In pursuance, then, of the plan above proposed, I shall consider in this
chapter, first, what may be the course of fluctuation in the animate
world; secondly, the mode in which contemporaneous subterranean
movements affect the earth's crust; and, thirdly, the laws which
regulate the deposition of sediment.

UNIFORMITY OF CHANGE CONSIDERED FIRST IN REFERENCE TO THE LIVING
CREATION.

First, in regard to the vicissitudes of the living creation, all are
agreed that the sedimentary strata found in the earth's crust are
divisible into a variety of groups, more or less dissimilar in their
organic remains and mineral composition. The conclusion universally
drawn from the study and comparison of these fossiliferous groups is
this, that at successive periods distinct tribes of animals and plants
have inhabited the land and waters, and that the organic types of the
newer formations are more analogous to species now existing, than those
of more ancient rocks. If we then turn to the present state of the
animate creation, and inquire whether it has now become fixed and
stationary, we discover that, on the contrary, it is in a state of
continual flux--that there are many causes in action which tend to the
extinction of species, and which are conclusive against