390 ORIGIN OF SPECIES 



On the theory of descent with modification, the great law 

 of the long enduring, but not immutable, succession of the 

 same types within the same areas, is at once explained; 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 distribution of 

 organic beings. 



It may be asked in ridicule, whether I suppose that the 

 megatherium and other allied huge monsters, which formerly 

 lived in South America, have left behind them 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 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 con- 

 clude 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 spe- 

 cies 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 



