Page - 347 - in The Origin of Species

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^ 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 of4 ^* 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