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. 



