e 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 compared with the average duration of specific forms;
that migration has played an important part in the first appear