IMPERFECTION OF GEOLOGICAL RECORD 73 



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 

 climatal and other changes; and when we see a species 

 first appearing in any formation, the probability is that 

 it only then first immigrated into that area. It is well 

 known, for instance, that several species appear somewhat 

 earlier in the paleozoic beds of North America than in 

 those of Europe; time having apparently been required 

 for their migration from the American to the European 

 seas. In examining the latest deposits in various quar- 

 ters 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 sur- 

 rounding sea; or, conversely, that some are now abun- 

 dant in the neighboring 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 during the glacial epoch, which forms only a 

 part of one whole geological period; and likewise to re- 

 flect 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 



