tinct, and have left no progeny. But in the caves
of Brazil there are many extinct species which are closely allied in
size and in all other characters to the species still living in South
America; and some of these fossils may have been the actual progenitors
of the living species. It must not be forgotten that, on our theory, all
the species of the same genus are the descendants of some one species;
so that, if six genera, each having eight species, be found in one
geological formation, and in a succeeding formation there be six other
allied or representative genera, each with the same number of species,
then we may conclude that generally only one species of each of the
older genera has left modified descendants, which constitute the new
genera containing the several species; the other seven species of each
old genus having died out and left no progeny. 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