It may be asked in ridicule, whether I suppose that the megatherium and
other allied huge monsters have left behind them in South America the
sloth, armadillo, and anteater, as their degenerate descendants. This
cannot for an instant be admitted. These huge animals have become
wholly extinct, and have left no progeny. But in the caves of Brazil,
there are many extinct species which are closely allied in size and in
other characters to the species still living in South America; and some
of these fossils may be the actual progenitors of living species. It
must not be forgotten that, on my theory, all the species of the same
genus have descended from some one species; so that if six genera, each
having eight species, be found in one geological formation, and in the
next succeeding formation there be six other allied or representative
genera with the same number of species, then we may conclude that only
one species of each of the six older genera has left modified
descendants, constituting the six new genera. The other seven species
of the old genera have all died out and have left no progeny. Or, which
would probably be a far commoner case, two or three species of two or
three alone of the six older genera will have been the parents of the
six new genera; the other old species and the other whole genera having
become utterly extinct. In failing orders, with the genera and species
decreasing in numbers, as apparently is the case of the Edentata of
South America, still fewer genera and species will have left 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
incalculable number of generations which must have passed away even
during a single formation; that, owing to subsidence being necessary
for the accumulation of fossiliferous deposits thick enough to resist
future degradation, enormous intervals of time have elapsed between the
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, perhaps, short
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