Or, and this will be a far commoner case, two or three species
in two or three alone of the six older genera will be the parents of the
new genera: the other species and the other old genera having become
utterly extinct. In failing orders, with the genera and species decreasing
in numbers as is the case with the Edentata of South America, still fewer
genera and species will leave modified blood-descendants.
SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.
I have attempted to show that the geological record is extremely imperfect;
that only a small portion of the globe has been geologically explored with
care; that only certain classes of organic beings have been largely
preserved in a fossil state; that the number both of specimens and of
species, preserved in our museums, is absolutely as nothing compared with
the number of generations which must have passed away even during a single
formation; that, owing to subsidence being almost necessary for the
accumulation of deposits rich in fossil species of many kinds, and thick
enough to outlast future degradation, great intervals of time must have
elapsed between most of our successive formations; that there has probably
been more extinction during the periods of subsidence, and more variation
during the periods of elevation, and during the latter the record will have
been least perfectly kept; that each single formation has not been
continuously deposited; that the duration of each formation is probably
short compared with the average duration of specific forms; that migration
has played an important part in the first appearance of new forms in any
one area and formation; that widely ranging species are those which have
varied most frequently, and have oftenest given rise to new species; that
varieties have at first been local; and lastly, although each species must
have passed through numerous transitional stages, it is probable that the
periods, during which each underwent modification, though many and long as
measured by years, have been short in comparison with the periods during
which each remained in an unchanged condition.
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