Chap. x. Absence of Intermediate Varieties, 275 



and granitic rocks exceed, in the proportion of 19 to 12*5, the whole 

 of the newer Palseozoic formations. In many regions the metamor- 

 phic and granitic rocks would be found much more widely extended 

 than they appear to be, if all the sedimentary beds were removed which 

 rest unconformably on them, and which 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 denuded, 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 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 

 will decrease (excepting 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 subsidence, 

 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 that the 

 geological record, viewed as a whole, is extremely imperfect ; but if 

 we confine our attention to any one formation, 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 Ammonites ; 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 com- 

 monly 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. 



Although each formation may mark a very long lapse of years, 

 each probably is short compared with the period requisite to change 

 one species into another. I am aware that two paleontologists, 

 whose opinions are worthy of much deference, namely Bronn and 



t 2 



