VIII CAUSES OF EXTINCTION 185 



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 

 another, filling the same place in the economy of nature, should 

 be abundant in a neighbouring district, differing very little in 

 its conditions. If asked how this is, one immediately 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 inappreciable by us, determine whether a given species 

 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 ^ 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 district — why should we feel 

 such great astonishment at the rarity being carried a step 

 farther to extinction ? An action going on, on every side of 

 us, and yet barely appreciable, might surely be carried a little 

 farther 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 



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

 Geology. 



