NATURAL SELECTION, ETC. 115 



gradual, climatic vicissitudes on land, which have 

 driven all tropical and subtropical forms out of the 

 higher latitudes and assigned to them their actual 

 limits, would be almost sure to extinguish such huge 

 and unwieldy animals as mastodons, mammoths, and 

 the like, whose power of enduring altered circum- 

 stances must have been small. 



This general replacement of the tertiary species 

 of a country by others so much like them is a note- 

 worthy fact. The hypothesis of the independent 

 creation of all species, irrespective of their antece- 

 dents, leaves this fact just as mysterious as is creation 

 itself ; that of derivation undertakes to account for it. 

 Whether it satisfactorily does so or not, it must be 

 allowed that the facts well accord with that hypothe- 

 sis. The same may be said of another conclusion, 

 namely, that the geological succession of animals and 

 plants appears to correspond in a general way with 

 their relative standing or rank in a natural system of 

 classification. It seems clear that, though no one of 

 the grand types of the animal kingdom can be traced 

 back farther than the rest, yet the lower classes long 

 preceded the higher; that there has been on the 

 whole a steady progression within each class and 

 order ; and that the highest plants and animals have 

 appeared only in relatively modern times. It is only, 

 however, in a broad sense that this generalization is 

 now thought to hold good. It encounters many ap- 

 parent exceptions, and sundry real ones. So far as 

 the rule holds, all is as it should be upon an hypothe- 

 sis of derivation. 



The rule has its exceptions. But, curiously enough, 



