THE VOYAGE OF THE BEAGLE 189 



species consumed the food of the great antecedent races? 

 Can we believe that the Capybara has taken the food of the 

 Toxodon, the Guanaco of the Macrauchenia, the existing 

 small Edentata of their numerous gigantic prototypes ? Cer- 

 tainly, no fact in the long history of the world is so startling 

 as the wide and repeated exterminations of its inhabitants. 



Nevertheless, if we consider the subject under another 

 point of view, it will appear less perplexing. We do not 

 steadily bear in mind, how profoundly ignorant we are o*f the 

 conditions of existence 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 na- 

 ture. 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 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 neighbour- 

 ing 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 pre- 

 cise 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: 



