Chap. X.] IN ANY SINGLE FORMATION. 67 



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 sev- 

 eral 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 climatal and other changes; and when we see a 

 species first appearing in any formation, the probabil- 

 ity is that it only then first immigrated into that area. 

 It is well-known, for instance, that several species ap- 

 pear somewhat earlier in the palaeozoic beds of North 

 America than in those of Europe; time having appa- 

 rently 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 deposit, but have become 

 extinct in the immediately surrounding sea; or, con- 

 versely, that some are now abundant in the neighbour- 

 ing sea, but are rare or absent in this particular deposit. 

 It is an excellent lesson to reflect on the ascertained 

 amount of migration of the inhabitants of Europe dur- 

 ing the glacial epoch, which forms only a part of one 

 whole geological period; and likewise to reflect on the 

 changes of level, on the extreme change of climate, and 

 on the great lapse of time, all included within this 

 same glacial period. Yet it may be doubted whether, 

 in any quarter of the world, sedimentary deposits, in- 

 30 



