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 species and what by a mere varietal form, and also on what we understand by creation and what we mean by development. Twenty years ago nearly all geologists were believers in creation, though it must be admitted without precisely understanding what they meant by the term. Now, the great impression produced by Darwin's speculations and the prevalence of the evolutionist philosophy have produced a leaning in the other direction. More recently, however, the absurdities into which the extreme evolutionists find themselves driven have produced a reaction; and we hope that views consistent with revelation, or at least with Theism, will again be in the ascendant, and that present controversies will serve to give more precise and definite views than heretofore of the relation of nature to God. As illustrations of the opinions prevalent before the rise of the development theory, I may quote from Pictet and Bronn, two of the most eminent palaeontologists.