318 SUMMAKY OF THE Cuap. X. 



ants. If llio inhabitants of one continent formerly differed 

 greatly from those of another continent, so will their modified 

 descendants still dillcr in nearly the same manner and degree, 

 lint after very long intervals of time, and after great geo- 

 graphical changes, permitting much inter-migration, the fee- 

 bler will yield to the more dominant forms, and there will be 

 nothing innnutable in the laws of past and present distribu- 

 tion. 



It may be asked in ridicule, whctlier I suppose tliat the 

 megatherium and other allied huge monsters, which formerly 

 lived in South America, have left behind them the sloth, arma- 

 dillo, and anteater, as their degenerate descendants. This can- 

 not for an instant be admitted. These huge animals have 

 become wlioll^'^ extinct, and have left no progeny. But in the 

 caves of Brazil, there are many extinct species Avhich are close- 

 ly allied in size and in all other characters to the species still 

 liv-ing in South America ; and some of these fossils may be the 

 actual progenitors of 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 repre- 

 sentative genera each Avith 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 several species of the new genera ; the other seven species 

 of each old genus ha^ang died out and left no progeny. Or, 

 and this probably 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 whole genera having become utterly extinct. In failing 

 ordcre, 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 andprese7it Chapter. 



I have attempted to show that the geological record is ex- 

 tremely imperfect ; that only a small portion of the globe has 

 been geologically explored with care ; that only certain class- 

 es of organic beings have been largely preserved in a fossil 

 state; that the number both of specimens and of species, pre- 

 served in our museums, is absolutely as notliing, compared 



