2 ON THE LAW WHICH HAS REGULATED 
inhabiting it, is but the last stage of a long and 
uninterrupted series of changes which it has under- 
gone, and consequently, that to endeavour to explain 
and account for its present condition without any 
reference to those changes (as has frequently been — 
done) must lead to very imperfect and erroneous 
conclusions. 
The facts proved by geology are briefly these :— 
That during an immense, but unknown period, the 
surface of the earth has undergone successive 
changes; land has sunk beneath the ocean, while 
fresh land has risen up from it; mountain chains 
have been elevated; islands have been formed into 
continents, and continents submerged till they have 
become islands; and these changes have taken place, 
not once merely, but perhaps hundreds, perhaps 
thousands of times :—That all these operations have 
been more or less continuous, but unequal in their 
progress, and during the whole series the organic 
life of the earth has undergone a corresponding 
alteration. This alteration also has been gradual, 
but complete; after a certain interval not a single 
species existing which had lived at the commence- 
ment of the period. This complete renewal of the 
forms of life also appears to have occurred several 
times :—That from the last of the geological epochs 
to the present or historical epoch, the change of 
organic life has been gradual: the first appearance 
of animals now existing can in many cases be traced, 
their numbers gradually increasing in the more re- 
