14 ON THE LAW WHICH HAS EEGULATED 



C. Lyell in his admirable " Principles." Geological 

 changes, however gradual, must occasionally have 

 modified external conditions to such an extent as 

 to have rendered the existence of certain species 

 impossible. The extinction would in most cases be 

 effected by a gradual dying-out, but in some in- 

 stances there might have been a sudden destruction 

 of a species of limited range. To discover how the 

 extinct species have from time to time been replaced 

 by new ones down to the very latest geological period, 

 is the most difficult, and at the same time the most 

 interesting problem in the natural history of the 

 earth. The present inquiry, which seeks to elimi- 

 nate from known facts a law which has determined, 

 to a certain degree, what species could and did ap- 

 pear at a given epoch, may, it is hoped, be consi- 

 dered as one step in the right direction towards a 

 complete solution of it. 



High Organization of very ancient Animals consistent 

 ivith this Law. 



Much discussion has of late years taken place on 

 the question, whether the succession of life upon the 

 globe has been from a lower to a higher degree of 

 organization. The admitted facts seem to show that 

 there has been a general, but not a detailed pro- 

 gression. Mollusca and Eadiata existed before Ver- 

 tebrata, and the progression from Fishes to Reptiles 

 and Mammalia, and also from the lower mammals 

 to the higher, is indisputable. On the other hand. 



