PAL^ONTO LOGICAL COLLECTIONS. 305 



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 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 previ- 

 ously 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, dur- 

 ing 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 VARIE- 

 TIES 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 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 pre- 

 senting 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 curi- 

 ous 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 i 

 given why each should not commonly include a graduated 

 series of links between the species which lived at its com- 

 mencement 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 requi- 

 site to change one species into another. I am aware that 

 two palaeontologists, whose opinions are worthy of much 



