In the Second Part{511} the direct evidence of allied forms having descended from the same stock was discussed. It was shown that this theory requires a long series of intermediate forms between the species and groups in the same classes--forms not directly intermediate between existing species, but intermediate with a common parent. It was admitted that if even all the preserved fossils and existing species were collected, such a series would be far from being formed; but it was shown that we have not _good_ evidence that the oldest known deposits are contemporaneous with the first appearance of living beings; or that the several subsequent formations are nearly consecutive; or that any one formation preserves a nearly perfect fauna of even the hard marine organisms, which lived in that quarter of the world. Consequently, we have no reason to suppose that more than a small fraction of the organisms which have lived at any one period have ever been preserved; and hence that we ought not to expect to discover the fossilised sub-varieties between any two species. On the other hand, the evidence, though extremely imperfect, drawn from fossil remains, as far as it does go, is in favour of such a series of organisms having existed as that required. This want of evidence of the past existence of almost infinitely numerous intermediate forms, is, I conceive, much the weightiest difficulty{512} on the theory of common descent; but I must think that this is due to ignorance necessarily resulting from the imperfection of all geological records. {511} Part II begins with Ch. IV. See the Introduction, where the absence of division into two parts (in the _Origin_) is discussed. {512} In the recapitulation in the last chapter of the _Origin_, Ed. i. p. 475, vi. p. 651, the author does not insist on this point as the weightiest difficulty, though he does so in Ed. i. p. 299. It is possible that he had come to think less of the difficulty in question: this was certainly the case when he wrote the 6th edition, see p. 438.