Of all the five biological sciences which have been called into evidence--viz. those of Classification, Morphology, Embryology, Palæontology, and Geographical Distribution--it is in the case of palæontology alone that any important or professional opinions still continue to be unsatisfied. Therefore, in order that justice may be done to this line of dissent, I have thought it better to deal with the matter in a separate Appendix, rather than to hurry it over in the text. And, as all the difficulties or objections which have been advanced against the theory of evolution on grounds of palæontology must vary, as to their strength, with the estimate which is taken touching the degree of imperfection of the geological record, I will begin by adding a few paragraphs to what has already been said in the text upon this subject. First, then, as to the difficulties in the way of fossils being formed at all. We have already noticed in the text that it is only the more or less hard parts of organisms which under any circumstances can be fossilized; and even the hardest parts quickly disintegrate if not protected from the weather on land, or from the water on the sea-bottom. Moreover, as Darwin says, "we probably take a quite erroneous view when we assume that sediment is being deposited over nearly the whole bed of the sea, at a rate sufficiently quick to embed and preserve fossil remains. Throughout an enormously large proportion of the ocean, the bright blue tint of the water bespeaks its purity. The many cases on record of a formation conformably covered, after an immense interval of time, by another and a later formation, without the underlying bed having suffered in the interval any wear and tear, seem explicable only on the view of the bottom of the sea not rarely lying for ages in an unaltered condition." Next, as regards littoral animals, he shows the difficulty which they must have in becoming fossils, and gives a striking example in several of the existing species of a sub-family of cirripedes (_Chthamalinæ_), "which coat the rocks all over the world in infinite numbers," yet, with the exception of one species which inhabits deep water, no vestige of any of them has been found in any tertiary formation, although it is known that the genus _Chthamalus_ existed through the Chalk period. Lastly, "with respect to the terrestrial productions which lived through the secondary and palæozoic periods, it is superfluous to state our evidence is fragmentary in an extreme degree. For instance, until recently not a land shell was known belonging to either of these vast periods," with one exception; while, "in regard to mammiferous remains, a glance at the historical table in Lyell's Manual will bring home the truth, how accidental and rare has been their preservation, far better than pages of detail. Nor is their rarity surprising, when we remember how large a proportion of the bones of tertiary mammals have been discovered either in caves or in