772 



THE AMERICAN NATURALIST 



[Vol. XL 



wSouth America. In South America and elsewhere (1) he does not 

 favor the extreme theory of the destructive influence of the Glacial 

 Epoch and he cites the supposed post-Glacial survival of Macrau- 

 chenia and Mastodon. "It could hardly have been a change of 

 temperature," he observes (p. 170), "which at about the same 

 time destroyed the inhabitants of tropical, temperate, and arctic 

 latitudes on both sides of the globe." (2) He dismisses the 

 possibility of extinction by man. (3) Also of an extended drought 

 in South America, calling attention to the Pampean horse as an 

 animal which could have survived a drought. 



In seeking to establish a general law of extinction Darwin 

 makes the following propositions: (1) the natural increase of 

 animals is in geometrical ratio, while, (2) the food supply remains 

 constant, thus (3) any great increase in numbers is impossible and 

 must be checked by some means. (4) We are seldom able to 

 state the cause of this check beyond saying that it is determined by 

 some slight difference in climate, food, or the number of enemies. 

 (5) We are, therefore, driven to the conclusion that causes gen- 

 erally quite inappreciable by us determine whether a given species 

 shall be abundant or scanty in numbers. (6) Comparative rarity 

 is the plainest evidence of less favorable conditions of existence. 

 (7) Rarity frequently precedes extinction, and if the too rapid 

 increase of species, even the most favored, is steadily checked, why 

 should we feel such great astonishment at the rarity being carried 

 a step farther to extinction. 



These were Darwin's earlier views expressed in The Voyage. 

 In The Origin of Species he discusses fully the checks to increase as 

 follows: (1) climate as directly unfavorable, (2) as indirectly 

 unfavorable by favoring other forms or by increasing the number of 

 certain competitors. (3) Unchecked increase frequently followed 

 by epidemics — possibly in part by facility of diffusion of parasites 

 amongst the crowded animals. (4) Finally, since a large stock 

 of individuals, relatively to the number of enemies, is absolutely 

 necessary for the preservation of a species a diminished number 

 would tend to extinction, (.y) Anv form fp. 133) which is repre- 

 sented by few indivi.luals will run a good cliance of utter extinction, 

 during great fluctuations in the nature of the seasons, or from a 

 temporary increase in the number of its enemies. (6) Diminution 



