CAUSES UF EXTiNCTIuX. 225 



ture regularly breeds ; yet in a species long estab- 

 lished, any great inci'ease in numbers is obviously 

 impossible, and must be checked by some means. 

 We are, nevertheless, seldom able with certainty 

 to tell, in any given species, at what period of life, 

 or at what period of the year, or whether only at 

 long intervals, the check falls ; or, again, what is 

 the precise nature of the check. Hence probably 

 it is that we feel so little surprise at one, of two 

 species closely allied in habits, being rare and the 

 other abundant in the same district ; or, again, that 

 one should be abundant in one district, and anoth- 

 er, filling the same place in the economy of nature, 

 should be abundant in a neighbouring district, dif- 

 fering very little in its conditions. If asked how 

 this is, one immediately replies that it is determin- 

 ed by some slight difference in climate, food, or the 

 number of enemies ; yet how rai'ely, if ever, we 

 can point out the precise cause and manner of ac- 

 tion of the check ! We are therefore driven to 

 the conclusion that causes generally quite inap- 

 preciable by us determine whether a given spe- 

 cies shall be abundant or scanty in numbers. 



In the cases where we can trace the extinction 

 of a species through man, either wholly or in one 

 limited district, we know that it becomes rarer and 

 rarer, and is then lost : it would be difficult to point 

 out any just distinction* betv/een a species destroy- 

 ed by man or by the increase of its natural enemies. 

 The evidence of rarity preceding extinction is more 

 striking in the successive tertiary strata, as remark- 

 ed by several able observers; it has often been found 

 that a shell very common in a tertiary stratum 

 is now most rare, and has even long been thought 

 to be extinct. If, then, as appears probable, spe- 



* See the excellent remarks on this subject by Mr Lyell, in his 

 Principles of Geology. 

 Vol, I — 15 



