EXTERMINATION IN ANIMAL LIFE. 341 



Accidents in field and flood are not uncommon. Thus with 

 the North American Bison, whose almost extirpation has been 

 caused by man, other agencies effected a thinning-out process. 

 It is stated that in 1867 upwards of two thousand Bison out of a 

 herd of four thousand were lost in a quicksand ; and that an 

 entire herd of about one hundred head perished when crossing 

 the ice on a lake in Minnesota. 



The almost complete destruction of the Cray-fish (Potamobia 

 Jiuviatilis) on the Thames was due to a disease, which first 

 appeared near Staines and worked its way up the river, " with 

 as much method as enteric fever worked its way down the Nile 

 in the Egyptian campaign after Omdurman." * Even insect 

 pests may thin out mammals. We read in the daily press that 

 a flock of sis thousand Sheep belonging to G. A. Brundrett, and 

 feeding on Matagorda Islands, Texas, were driven by Mosquitoes 

 into the water of Cedar Canon. All but four hundred were 

 drowned. The island is so infested (1902) with Mosquitoes that 

 families have had to abandon their homes. t 



On the other hand, it seems very evident that after unfavour- 

 able seasons which may have proved inimical to animal life, 

 other circumstances may arrive which considerably add to the 

 fecundity of species, and thus much loss may be repaired. 

 There are facts to support this view. The Short-eared Owl 

 (Asio accipitrinus) , whose number of eggs is generally four, has 

 been known when food is unusually abundant, as during a 

 Lemming migration, to produce a clutch of seven or eight ; and 

 during a recent Vole plague in Scotland, in some instances as 

 many as thirteen eggs were recorded. A similar instance has 

 been narrated with reference to Trout at Vermont, North 

 America. In a pond formed by damming a small stream to 

 obtain water power for a saw-mill, and covering one thousand 

 acres of primitive forest, the increased supply of food brought 

 within reach of the fish multiplied them to that degree that, at 

 the head of the pond where in the spring they crowded together 

 in the brook, which supplied it, they were taken in the hands at 

 pleasure, and swine caught them without difficulty. A single 

 sweep of a small scoop-net would bring up half a bushel, carts 



* Cornish, ' Naturalist on the Thames,' p. 52. 

 f ' Express,' October, 1902. 



