190 CHARLES DARWIN 



it would be difficult to point out any just distinction 11 

 between a species destroyed by man or by the increase of its 

 natural enemies. The evidence of rarity preceding extinc- 

 tion, is more striking in the successive tertiary strata, as 

 remarked 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, species first become rare and then 

 extinct if the too rapid increase of every species, even the 

 most favoured, is steadily checked, as we must admit, though 

 how and when it is hard to say and if we see, without the 

 smallest surprise, though unable to assign the precise reason, 

 one species abundant and another closely allied species rare 

 in the same district why should we feel such great aston- 

 ishment at the rarity being carried a step further to extinc- 

 tion ? An action going on, on every side of us, and yet barely 

 appreciable, might surely be carried a little further, without 

 exciting our observation. Who would feel any great sur- 

 prise at hearing that the Magalonyx was formerly rare com- 

 pared with the Megatherium, or that one of the fossil mon- 

 keys was few in number compared with one of the now 

 living monkeys? and yet in this comparative rarity, we 

 should have the plainest evidence of less favourable condi- 

 tions for their existence. To admit that species generally 

 become rare before they become extinct to feel no surprise 

 at the comparative rarity of one species with another, and 

 yet to call in some extraordinary agent and to marvel greatly 

 when a species ceases to exist, appears to me much the same 

 as to admit that sickness in the individual is the prelude 

 to death to feel no surprise at sickness but when the sick 

 man dies to wonder, and to believe that he died through 

 violence. 



13 See the excellent remarks on this subject by Mr. Lyell, in his Principles 

 of Geology. 



