But while rejecting transmutation, I was equally opposed to the popular theory that the creative power had diminished in energy, or that it had been in abeyance ever since Man had entered upon the scene. That a renovating force which had been in full operation for millions of years should cease to act while the causes of extinction were still in full activity, or even intensified by the accession of Man's destroying power, seemed to me in the highest degree improbable. The only point on which I doubted was whether the force might not be intermittent instead of being, as Lamarck supposed, in ceaseless operation. Might not the births of new species, like the deaths of old ones, be sudden? Might they not still escape our observation? If the coming in of one new species, and the loss of one other which had endured for ages, should take place annually, still, assuming that there are a million of animals and plants living on the globe, it would require, I observed, a million of years to bring about a complete revolution in the fauna and flora. In that case, I imagined that, although the first appearance of a new form might be as abrupt as the disappearance of an old one, yet naturalists might never yet have witnessed the first entrance on the stage of a large and conspicuous animal or plant, and as to the smaller kinds, many of them may be conceived to have stolen in unseen, and to have spread gradually over a wide area, like species migrating into new provinces.* (* "Principles of Geology" 1st edition 1832 volume 2 chapter 11; and 9th edition page 706.) It may now be useful to offer some remarks on the very different reception which the twin branches of Lamarck's development theory, namely, progression and transmutation, have met with, and to inquire into the causes of the popularity of the one and the great unpopularity of the other. We usually test the value of a scientific hypothesis by the number and variety of the phenomena of which it offers a fair or plausible explanation. If transmutation, when thus tested, has decidedly the advantage over progression and yet is comparatively in disfavour, we may reasonably suspect that its reception is retarded, not so much by its own inherent demerits, as by some apprehended consequences which it is supposed to involve and which run counter to our preconceived opinions. THEORY OF PROGRESSION. In treating of this question, I shall begin with the doctrine of progression, a concise statement of which, so far as it relates to the animal kingdom, was thus given twelve years ago by Professor Sedgwick, in the preface to his "Discourse on the Studies of the University of Cambridge."