for the inhabitants of each quarter of the
world will obviously tend to leave in that quarter, during the next
succeeding period of time, closely allied though in some degree modified
descendants. If the inhabitants of one continent formerly differed greatly
from those of another continent, so will their modified descendants still
differ in nearly the same manner and degree. But after very long intervals
of time and after great geographical changes, permitting much
inter-migration, the feebler will yield to the more dominant forms, and
there will be nothing immutable in the laws of past and present
distribution. {341}

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 old 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 {342}
only certain classes of organic beings have been l