ABSENCE OF INTERMEDIATE VARIETIES 347 



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 defer- 

 ence, namely Bronn and Woodward, have concluded that the 

 average duration of each formation is twice or thrice as long 

 as the average duration of specific forms. But insuperable 

 difficulties, as it seems to me, prevent us from coming to any 

 just conclusion on this head. When we see a species first 

 appearing in the middle of any formation, it would be rash 

 in the extreme to infer that it had not elsewhere previously 

 existed. So again when we find a species disappearing before 

 the last layers have been deposited, it would be equally rash 

 to suppose that it then became extinct. We forget how small 

 the area of Europe is compared with the rest of the world; 

 nor have the several stages of the same formation throughout 

 Europe been correlated with perfect accuracy. / 



We may safely infer that with marine animals of all kinds] ^ 

 there has been a large amount of migration due to climatalj ^^j^S 

 and other changes; and when we see a species first appearing\^ 

 in any formation, the probability is that it only then first im- 

 migrated into that area. It is well known, for instance, that 

 several species appear somewhat earlier in the palaeozoic beds 

 of North America than in those of Europe ; time having ap- 

 parently been required for their migration from the American 

 to the European seas. In examining the latest deposits in 

 various quarters of the world, it has everywhere been noted, 

 that some few still existing species are common in the de- 

 posit, but have become extinct in the immediately surround- 

 ing sea ; or, conversely, that some are now abundant in the 

 neighbouring sea, but are rare or absent in this particular 

 deposit. It is an excellent lesson to reflect on the ascer^ v 

 tained amount of migration of the inhabitants of Europe dur- I *^ 

 ing the glacial epoch, which forms only a part of one whole J "^ 

 geological period; and likewise to reflect on the changes of 4 ^* 

 level, on the extreme change of climate, and on the great "1^^ *' 

 lapse of time, all included within this same glacial period. I .v--^ 

 Yet it may be doubted whether, in any quarter of the world, 

 sedimentary deposits, including fossil remains, have gone on 

 accumulating within the same area during the whole of this 



