The interest for the general reader heightens as the author advances on his perilous way and grapples manfully with the most formidable difficulties.

To account, upon these principles, for the gradual elimination and segregation of nearly allied forms—­such as varieties, sub-species, and closely-related or representative species—­also in a general way for their geographical association and present range, is comparatively easy, is apparently within the bounds of possibility.  Could we stop here we should be fairly contented.  But, to complete the system, to carry out the principles to their ultimate conclusion, and to explain by them many facts in geographical distribution which would still remain anomalous, Mr. Darwin is equally bound to account for the formation of genera, families, orders, and even classes, by natural selection.  He does “not doubt that the theory of descent with modification embraces all the members of the same class,” and he concedes that analogy would press the conclusion still further; while he admits that “the more distinct the forms are, the more the arguments fall away in force.”  To command assent we naturally require decreasing probability to be overbalanced by an increased weight of evidence.  An opponent might plausibly, and perhaps quite fairly, urge that the links in the chain of argument are weakest just where the greatest stress falls upon them.

To which Mr. Darwin’s answer is, that the best parts of the testimony have been lost.  He is confident that intermediate forms must have existed; that in the olden times when the genera, the families, and the orders, diverged from their parent stocks, gradations existed as fine as those which now connect closely related species with varieties.  But they have passed and left no sign.  The geological record, even if all displayed to view, is a book from which not only many pages, but even whole alternate chapters, have been lost out, or rather which were never printed from the autographs of Nature.  The record was actually made in fossil lithography only at certain times and under certain conditions (i.e., at periods of slow subsidence and places of abundant sediment); and of these records all but the last volume is out of print; and of its pages only local glimpses have been obtained.  Geologists, except Lyell, will object to this—­some of them moderately, others with vehemence.  Mr. Darwin himself admits, with a candor rarely displayed on such occasions, that he should have expected more geological evidence of transition than he finds, and that all the most eminent paleontologists maintain the immutability of species.