I shall proceed in the following chapters to enumerate some of the principal difficulties still opposed to the theory of the uniform nature and energy of the causes which have worked successive changes in the crust of the earth, and in the condition of its living inhabitants. The discussion of so important a question on the present occasion may appear premature, but it is one which naturally arises out of a review of the former history of the science. It is, of course, impossible to enter into such speculative topics, without occasionally carrying the novice beyond his depth, and appealing to facts and conclusions with which he will be unacquainted, until he has studied some elementary work on geology, but it may be useful to excite his curiosity, and lead him to study such works by calling his attention at once to some of the principal points of controversy.[125] CHAPTER VI. DOCTRINE OF THE DISCORDANCE OF THE ANCIENT AND MODERN CAUSES OF CHANGE CONTROVERTED. Climate of the Northern Hemisphere formerly different--Direct proofs from the organic remains of the Italian strata--Proofs from analogy derived from extinct quadrupeds--Imbedding of animals in icebergs--Siberian mammoths--Evidence in regard to temperature, from the fossils of tertiary and secondary rocks--From the plants of the coal formation--Northern limit of these fossils--Whether such plants could endure the long continuance of an arctic night. _Climate of the Northern hemisphere formerly different._--Proofs of former revolutions in climate, as deduced from fossil remains, have afforded one of the most popular objections to the theory which endeavors to explain all geological changes by reference to those now in progress on the earth. The probable causes, therefore, of fluctuations in climate, may first be treated of.