1834.1 CAUSES OF EXTINCTION. 175 



view, it will appear less perplexing. We do not steadily bear in 

 mind, how profoundly ignorant we are of the conditions of exist- 

 ence of every animal ; nor do we always remember, that some 

 check is constantly preventing the too rapid increase of every 

 organized being left in a state of nature. The supply of food, on 

 an average, remains constant ; yet the tendency in every animal to 

 increase by propagation is geometrical ; and its surprising effects 

 have nowhere been more astonishingly shown, than in the case 

 of the European animals run wild during the last few centuries 

 in America. Every animal in a state of nature regularly breeds ; 

 yet in a species long established, any great increase 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 surpidse at one, of two species closely allied in 

 habits, being rare tind the other abundant in the same district ; 

 or, again, that one sliould 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. Tlie 

 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 tertiaiy 

 stratum is now most rare, and has even long been thought to l)e 



* See the excellent remarks on this subject by Mr. Tiyell, in his Prin- 

 ciples of Geology. 



