1834.] CAUSES OF EXTINCTION. 167 



that it becomes rarer and rarer, and is then lost: it would be 

 difficult to point out any just distinction * between a species 

 destroyed 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 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 dis- 

 trict why should we feel such great astonishment at the rarity 

 being carried a step further to extinction ? 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 surprise at hearing that the Megalonyx was 

 formerly rare compared with the Megatherium, or that one of the 

 fossil monkeys 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 conditions 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. 



* See the excellent remarks ou this subject by Mr. Lyell, in his Prin- 

 ciples of Geology. 



