132 THE WORLD OF LIFE 



ilarly 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 dis- 

 ease or the attacks of enemies. 



In Mr. W. II. Hudson's most interesting volume, A Nat- 

 uralist 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 pas- 

 sage 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 pred- 

 atory 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 man}^ 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 l^ature's silent, passionless tragedies, myriads of highly 

 organised beings rising into existence only to perish almost imme- 

 diately, scarcely a hard-pressed remnant surviving to continue the 

 species." 



It may, however, be concluded that not thus are species 

 exterminated in any region that remains suitable for their 

 existence. Long before they approach extinction, the very 

 scarcity of them drives away, one after another, the crowd of 



