But, as a matter of fact, the amount of convergence of series, in relation to the time occupied by the deposition of geological formations, is extraordinarily small.  Of all animals the higher Vertebrata are the most complex; and among these the carnivores and hoofed animals (Ungulata) are highly differentiated.  Nevertheless, although the different lines of modification of the Carnivora and those of the Ungulata, respectively, approach one another, and, although each group is represented by less differentiated forms in the older tertiary rocks than at the present day, the oldest tertiary rocks do not bring us near the primitive form of either.  If, in the same way, the convergence of the varied forms of reptiles is measured against the time during which their remains are preserved—­which is represented by the whole of the tertiary and mesozoic formations—­the amount of that convergence is far smaller than that of the lines of mammals between the present time and the beginning of the tertiary epoch.  And it is a broad fact that, the lower we go in the scale of organization, the fewer signs are there of convergence towards the primitive form from whence all must have diverged, if evolution be a fact.  Nevertheless, that it is a fact in some cases, is proved, and I, for one, have not the courage to suppose that the mode in which some species have taken their origin is different from that in which the rest have originated.

What, then, has become of all the marine animals which, on the hypothesis of evolution, must have existed in myriads in those seas, wherein the many thousand feet of Cambrian and Laurentian rocks now devoid, or almost devoid, of any trace of life were deposited?

Sir Charles Lyell long ago suggested that the azoic character of these ancient formations might be due to the fact that they had undergone extensive metamorphosis; and readers of the “Principles of Geology” will be familiar with the ingenious manner in which he contrasts the theory of the Gnome, who is acquainted only with the interior of the earth, with those of ordinary philosophers, who know only its exterior.

The metamorphism contemplated by the great modern champion of rational geology is, mainly, that brought about by the exposure of rocks to subterranean heat; and where no such heat could be shown to have operated, his opponents assumed that no metamorphosis could have taken place.  But the formation of greensand, and still more that of the “red clay” (if the Challenger hypothesis be correct) affords an insight into a new kind of metamorphosis—­not igneous, but aqueous—­by which the primitive nature of a deposit may be masked as completely as it can be by the agency of heat.  And, as Wyville Thomson suggests, in the passage I have quoted above (p. 17), it further enables us to assign a new cause for the occurrence, so puzzling hitherto, of thousands of feet of unfossiliferous fine-grained schists and