273 ABSENCE OF INTERMEDIATE VARIETIES Chap. IX. 



of 19 to 1:2.5, the Avholc of the newer Paleozoic formations. 

 In many rcg'ions the metamorphic and granitic rocks would be 

 seen to be much more widely extended, if all the sedimentary 

 beds Averc removed which rest unconformably on them, and 

 whicli could 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 peri- 

 ods 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 favorable, as previously explained, 

 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 Avill decrease (excepting on the shores 

 of a continent when fii-st broken up into an archipelago), and 

 consequently during subsidence, though there will bo much ex- 

 tinction, few new varieties or species will be formed ; and it 

 is during these very periods of subsidence that the deposits 

 which are richest in fossUs have been accumulated. 



On the Absence of JVumcrous Intermediate Varieties in any 

 one Single Formatio7i, 



From these several considerations, it cannot be doubted 

 that the geological record, viewed as a whole, is extremely im- 

 perfect ; but if we conline our attention to any one formation, 

 it becomes much more diilicult 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 arc on record of the same species presenting va- 

 rieties in the upper and lower parts of the same formation : 

 thus, Trautschold gives a number of instances with Ammon- 

 ites ; and Hilgendorf has described a most curious case of ten 

 graduated forms of Planorbis multiformis in the successive 

 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 connnonly include a graduated series of links be- 

 tween the species which lived at its commencement and close; 

 but I cannot assign due proportional weight to the following 

 considerations. 



