lacustrine deposits; and that not a cave or true lacustrine bed is known belonging to the age of our secondary or palæozoic formations." But perhaps of even more importance than all these known causes which prevent the formation of fossils, is the existence of unknown causes which make for the same result. For example, the Flysch-formation is a formation of several thousand feet in thickness (as much as 6000 in some places), and it extends for at least 300 miles from Vienna to Switzerland; moreover, it consists of shale and sandstone. Therefore, alike in respect of time, space, and character, it is just such a formation as we should expect to find highly rich in fossils; yet, "although this great mass has been most carefully searched, no fossils, except a few vegetable remains, have been found." So much then for the difficulty, so to speak, which nature experiences in the manufacture of fossils. Probably not one per cent. of the species of animals which have inhabited the earth has left a single individual as a fossil, whereby to record its past existence. But of even more importance than this difficulty of making fossils in the first instance, is the difficulty of preserving them when they are made. The vast majority of fossils have been formed under water, and a large proportional number of these--whether the animals were marine, terrestrial, or inhabitants of fresh water--have been formed in sedimentary deposits either of sand, gravel, or other porous material. Now, where such deposits have been afterwards raised into the air for any considerable time--and this has been more or less the case with all deposits which are available for exploration--their fossiliferous contents will have been, as a general rule, dissolved by the percolation of rain-water charged with carbonic acid. Similarly, sea-water has recently been found to be a surprisingly strong solvent of calcareous material: hence, Saturn-like, the ocean devours her own progeny as far as shells and bones of all kinds are concerned--and this to an extent of which we have probably no adequate conception.