CHAP. I.] 
INTRODUCTORY. 
7 
slow process of development or transmutation, all animals have 
been produced from those which preceded them ; and the old 
notion that every species was specially created as they now 
exist, at a particular time and in a particular spot, is abandoned 
as opposed to many striking facts, and unsupported by any 
evidence. This modification of animal forms took place very 
slowly, so that the historical period of three or four thousand 
years has hardly produced any perceptible change in a single 
species. Even the time since the last glacial epoch, which on 
the very lowest estimate must be from 50,000 to 100,000 years, 
has only served to modify a few of the higher animals into very 
slightly different species. The changes of the forms of animals 
appear to have accompanied, and perhaps to have depended 
on, changes of physical geography, of climate, or of vegetation ; 
since it is evident that an animal which is well adapted to one 
condition of things will require to be slightly changed in con- 
stitution or habits, and therefore generally in form, structure, or 
colour, in order to be equally well adapted to a changed 
condition of surrounding circumstances. Animals multiply so 
rapidly, that we may consider them as continually trying to 
extend their range ; and thus any new land raised above the 
sea by geological causes becomes immediately peopled by a 
crowd of competing inhabitants, the strongest and best adapted 
of which alone succeed in maintaining their position. 
If we keep in view these facts — that the minor features of the 
earth’s surface are everywhere slowly changing; that the forms, 
and structure, and habits of all living things are also slowly 
changing ; while the great features of the earth, the continents, 
and oceans, and loftiest mountain ranges, only change after very 
long intervals and with extreme slowness; we must see that 
the present distribution of animals upon the several parts of 
the earth’s surface is the final product of all these wonderful 
revolutions in organic and inorganic nature. The greatest and 
most radical differences in the productions of any part of the 
globe must be dependent on isolation by the most effectual 
and most permanent barriers. That ocean which has remained 
broadest and deepest from the most remote geological epoch 
