THE STRUGGLE FOR EXISTENCE 79 



good cause for leaving. The winter had been 

 one of continued drought ; the dry grass and 

 herbage of the preceding year had been consumed 

 by the cattle and wild animals, or had turned 

 to dust, and, with the disappearance of their 

 food and cover, the mice had ceased to be." The 

 cats sneaked back to the houses. " It was pitiful 

 to see the little burrowing owls ; for these birds, 

 not having the powerful wings and prescient 

 instincts of the vagrant Otus brachyotus, were 

 compelled to face the poverty from which the 

 others escaped." They became tame with hunger, 

 and so reduced as scarcely to be able to fly. 



Fine weather, ready cover, and plenty of food 

 had allowed the mice to multiply beyond measure, 

 but their enemies had likewise increased. As 

 the herbage disappeared, multipUcation of mice 

 ceased, and the army of enemies cleared off the 

 residue so thoroughly that "in spring it was 

 hard to find a survivor, even in barns and houses." 



This " wave of life " is one of the most in- 

 structive of biological pictures. It illustrates the 

 web of life, and the variety of the struggle for 

 existence. A physical change lets the stream of 

 life overflow, and, as the flood gathers momentum, 

 it widens the breach in its banks. One struggle 

 causes another struggle. Flowers abound, bees 

 abound, mice abound, cats and owls aboimd, and 

 there is struggle amongst all. Diets are changed, 

 habits are changed, numerical proportions are 

 changed, and then the season changes and all 

 is over. The mice are reduced to a minimum 

 and the wave of life is lost in the sand. 



" The fact," Mr. Hudson says, " that species 

 tend to increase in a geometrical ratio makes 



