Chap. X. 
SAME TYPES IN SAME AREAS. 
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 ani¬ 
mals have become wholly extinct, and have left no pro¬ 
geny. 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 for¬ 
gotten 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 forma¬ 
tion there be six other allied or representative genera 
with the same number of species, then we may con¬ 
clude 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 
