Page - 346 - in The Origin of Species

346 ORIGIN OF SPECIES not have formed part of the original mantle under which they were crystallized. Hence it is probable that in some parts of the world whole formations have been completely de- nuded, with not a wreck left behind. One remark is here worth a passing notice. During periods of elevation the area of the land and of the adjoining shoal parts of the sea will be increased, and new stations will often be formed :—all circumstances favourable, as previously ex- plained, for the formation of new varieties and species; but during such periods there will generally be a blank in the geological record. On the other hand, during subsidence, the inhabited area and number of inhabitants will decrease (ex- cepting on the shores of a continent when first broken up into an archipelago), and consequently during subsidence, though there will be much extinction, few new varieties or species will be formed; and it is during these very periods of subsi- dence, that the deposits which are richest in fossils have been accumulated. ON THE ABSENCE OF NUMEROUS INTERMEDIATE VARIETIES IN ANY SINGLE FORMATION / From these several considerations, it cannot be doubted i that the geological record, viewed as a whole, is extremely I imperfect; but if we confine our attention to any one forma- tion, it becomes much more difficult to understand why we do not therein find closely graduated varieties between the allied species which lived at its commencement and at its close. Several cases are on record of the same species presenting varieties in the upper and lower parts of the same formation ; thus, Trautschold gives a number of instances with Ammo- nites ; and Hilgendorf has described a most curious case of ten graduated forms of Planorbis multiformis in the succes- sive beds of a fresh-water formation in Switzerland. Although each formation has indisputably required a vast number of years for its deposition, several reasons can be given why each should not commonly include a graduated series of links between the species which lived at its commencement and close; but I cannot assign due proportional weight to the following considerations.