3:>6 ABSENCE OF INTERMEDIATE VARIETIES 



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 palasontolo^ists, whose opinions are worthy of 

 much deference, namely Bronn and Woodward, have con- 

 cluded 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 forma- 

 tion, it would be rash in the extreme to infer that it had 

 not elsewhere previously existed. 80 again, when we find 

 a species disappearing before the last layers have been de- 

 posited, 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 safeW 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 

 apiDcaring 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 palaeozoic 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 quarters of the 

 world, it lias everywhere been noted, that some few still 

 existing species are common in the deposit, but have 

 become extinct in the immediately surrounding sea; or, 

 conversely, that some are now abundant in the neighbor- 

 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 during 

 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 



