IS34-] CAUSES OF EXTINCTION. 12? 



diately replies that it is determined by some slight difference in climate, 

 food, or the number of enemies : yet how rarely, if ever, we can point 

 out the precise cause and manner of action of the check ! We are, 

 therefore, driven to the conclusion, that causes generally quite inappre- 

 ciable by us, determine whether a given species shall be abundant or 

 scanty in numbers. 



In the cases where \ve 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 * 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 extinc' "-*-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 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 com- 

 pared 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 on this subject by Mr, Lye.Ur *B his " Prin- 

 ciples of Geology." 



