volutions of the globe have involved no change of the
general laws of matter; and though it is possible that geology has
carried us back to the time when the laws that regulate life began to
operate, it does not show that they were less perfect than now, and it
indicates no trace of the beginning of the inorganic laws. Geological
changes have resulted not from the institution of new laws, but from
new _dispositions_, under existing laws and general arrangements.
There is every reason to believe that in the inorganic world these
dispositions have required no new creative interpositions during the
time to which geology refers, but merely the continued action of the
properties bestowed on matter when first produced. In the organic
world the case is different.

7. In the succession of animal and vegetable life we find a constant
improvement and advance by the introduction of new types of being. We
have already given a general outline of this advancement of organized
nature. It has consisted in the introduction, from time to time, of
new and more highly organized beings, so as at once to increase the
variety of nature, and to provide for the elevation of the summit of
the graduated scale of life to higher and higher points. At the same
time, in each successive period, it has been the law of creation that
the forms of life then dominant should attain their highest
development, and should then be succeeded by more advanced types. For
instance, in the earlier Palaeozoic period we have molluscous animals
and fishes, then apparently the highest forms of life, appearing with
a very advanced organization, not surpassed, if even equalled, in
modern times. In the latter part of the same period, some lower forms
of vegetable life, now restricted to a comparatively humble place,
were employed to constitute magnificent forests. In the Mesozoic
period, again, reptiles attained to their highest point in
organization and variety of form and employment, while mammalia had as
yet scarcely appeared.[146]

8. If now we ask in what manner the succession of life on the earth
has been produced, two apparently opposite hypotheses rise before us.
The one is that of introduction of new species by creative acts, the
other that of development of new species by changes of those
previously existing. In one respect the difference of these views is
little more than one of expression, for the meaning of the statements
depends on what we understand by a s