If we, therefore, here disregard all objections to the Doctrine of Descent which may be raised by the priests of the different religious faiths, we must nevertheless endeavour to refute the most important of those objections which seem more or less founded on science, and which we grant might, at first sight, to a certain extent captivate us and deter us from adopting the Doctrine of Descent. Many persons seem to think the length of the periods of time required the most important of these objections. We are not accustomed to deal with such immense periods as are necessary for the history of the creation. It has already been mentioned that the periods, during which species originated by gradual transmutation, must not be calculated by single centuries, but by hundreds and by millions of centuries. Even the thickness of the stratified crust of the earth, the consideration of the immense space of time which was requisite for its deposition from water, taken together with the periods of elevation between the periods of depression, indicate a duration of time of the organic history of the earth which the human intellect cannot realize. We are here in much the same position as an astronomer in regard to infinite space. In the same way as the distances between the different planetary systems are not calculated by miles but by Sirius-distances, each of which comprises millions of miles, so the organic history of the earth must not be calculated by thousands of years, but by palæontological or geological periods, each of which comprises many thousands of years, and perhaps millions, or even, milliards, of thousands of years. It is of little importance how high the immeasurable length of these periods may be approximately estimated, because we are in fact unable with our limited power of imagination to form a true conception of these periods, and because we do not as in astronomy possess a secure mathematical basis for fixing the approximate length of duration in numbers. But we most positively deny that we see any objection to the theory of development in the extreme length of these periods which are so completely beyond the power of our imagination. It is, on the contrary, as I have already explained in one of the preceding chapters, most advisable, from a strictly philosophical point of view, to conceive these periods of creation to be as long as possible, and we are by so much the less in danger of losing ourselves in improbable hypotheses, the longer we conceive the periods for organic processes of development to have been. The longer, for example, we conceive the Permian period to have been, the easier it will be for us to understand how the important transmutations took place within it which so essentially distinguish the fauna and flora of the Coal period from that of the Trias. The great disinclination which most persons have to assume such immeasurable periods, arises mainly from the fact of our having in early youth been