122 THE WORLD OF LIFE 



Those that finally perish in the sea are only acting under the same 

 blind impulse which has led them previously to cross smaller pieces 

 of water with safety." 



The strange history of these small creatures, besides 

 showing the enormous powers of increase in various types 

 of life, also furnishes us with a fine example of adaptation 

 to what would be, to most animals, extremely adverse con- 

 ditions — high plateaux within or bordering on the Arctic 

 circle, with its intense cold, its long periods of darkness, 

 buried in snow in winter, and with a scanty and stunted 

 vegetation. Yet they appear to have a most enjoyable 

 existence, and would evidently be able to overrun and 

 occupy a much larger extent of similarly inhospitable 

 country did such exist in their vicinity ; while in more 

 fertile lands, with a milder climate and more luxuriant 

 vegetation, they rapidly become extinct through disease or 

 the attacks of enemies. 



In Mr. W. H. Hudson's most interesting volume, A 

 Naturalist in La Plata, he gives an account of a very 

 similar rapid increase of field-mice, under extremely different 

 conditions, in the chapter entitled A Wave of Life. In 

 a concluding passage he so clearly summarises the whole 

 course of events that I here extract it : — 



" Cover and food without limit enabled the mice to increase at 

 such an amazing rate, that the ordinary checks interposed by pre- 

 datory species were for a while inappreciable. But as the mice 

 increased so did their enemies. Insectivorous and other species 

 acquired the habits of owls and weasels, preying exclusively on 

 them ; while to this an innumerable array of residents was shortly 

 added multitudes of wandering birds coming from distant regions. 

 No sooner had the herbage perished, depriving the little victims of 

 their cover and food, than the effects of the war became apparent. 

 In autumn the earth so teemed with them that one could scarcely 

 walk anywhere without treading on mice ; while out of every hollow 

 weed-stalk lying on the ground dozens could be shaken ; but so 

 rapidly had they been devoured by the trained army of persecutors 

 that in spring it was hard to find a survivor even in the barns and 

 houses. The fact that species tend to increase in a geometrical ratio 

 makes these great and sudden changes frequent in many parts of the 

 earth ; but it is not often that they present themselves so vividly 

 as in the foregoing instance, for here, scene after scene in one of j 

 Nature's silent, passionless tragedies, myriads of highly organised 



