254 PROCEEDINGS OF THE ACADEMY OF [1895. 



not destruction, being its purpose. The only animals which act 

 largely together in the work of destruction are the ants, yet with all 

 their intelligent combination for this purpose it is almost certain that 

 no species of insect owes its extinction to ant aggression. Among exist- 

 ing animals there are certain carnivorous fishes whose destruction of 

 other, helplessspecies is annually enormous, yet these depleted species 

 far from disappearing, return each year in vast multitudes to their 

 feeding grounds. 80 far as existing evidence goes, then, it seems 

 probable that hostile aggression, while it may have occasionally been 

 an indirect, has rarely been the direct cause of the extinction of 

 species. 



It is equally doubtful if extinction has been due, as a general 

 rule, to lack of suitable food. This may have been the case with 

 certain invertebrates adapted to very narrow food conditions, and 

 with some highly specialized vertebrates, confined to a transitory 

 condition of the food-supply. But ordinarily the food-supply, at least 

 of vertebrates, is wide- spread and persistent, while most of the higher 

 animals have some power of variation in this respect, and can adapt 

 themselves to new kinds of food. It would seem most probable, on 

 the whole, that extinction of species has been generally due to indirect 

 rather than to direct influences. Species of animals and plants have 

 rarely, if ever, disappeared through their destruction by other species 

 as food, and rarely through a natural insufficiency of food. Most 

 probably the usual causes of destruction have been adverse con- 

 ditions of nature, and the competition of other species in the struggle 

 for food. 



Nature undoubtedly has been active in this work, her adverse 

 influences being violent ami wide-spread; storms, sudden and severe 

 changes in temperature, long- continued floods, extensive droughts, 

 and occasionally highly destructive volcanic or other convulsions. 

 To such influences entire species may in some instances have 

 succumbed, particularly where the adverse conditions were of long 

 continuance, while other species may have been so greatly reduced in 

 numbers and energy as to become incapable of sustaining themselves 

 against the competition of more vigorous rivals. Among recent 

 instances of this kind may be classed the destruction of large 

 numbers of cattle and other domestic animals on the western plains 

 iu winters of great severity and deep snow-fall. This destruction 

 takes place despite all the efforts of Man to prevent it, and would be 



