200 LIFE : OUTLINES OF GENERAL BIOLOGY 



rodents called Lemmings —like small editions of Guinea-pigs; and 

 every four years or so there is an over-population crisis. The lem- 

 mings, having outrun the means of subsistence — devoured all the 

 vegetation, in fact- go on a march, from which there is no return. 

 I^rge numbers are found drowned on the shores of the Baltic and 

 the North Sea, and most of the trekkers come to grief in other ways. 

 Yet after a couple of years things are once more very much as they 

 were. The balance has been restored. In some cases in the past 

 similar crises in the history of other animals have proved too serious, 

 and s|)ecies have been exterminated; but even more striking is the 

 tendency that things have to right themselves. 



A Wave of Life. — In his Naturalist in La Plata, \V. H. Hudson 

 tells of the summer 1872-3 that it was rich in sunshine and showers, 

 blossoms and bees. The season was also very favourable for mice, 

 which devoured the bees, and became so numerous that one could 

 scarcely walk an}'^vhere without treading on them. Cats became wild 

 hunters; dogs ate almost nothing but mice; foxes, weasels, and 

 ojx)ssums fared sumptuously; tyTant-birds, Guira cuckoos, and even 

 fowls became mouse-eaters. Countless numbers of storks and short- 

 eared owls came to assist at the general feast. But the winter was 

 one of continued drought; the herbage was consumed or turned to 

 dust and, with the disappearance of their food and cover, the mice 

 ceased to be. The army of enemies, now in retreat, cleared of! the 

 residue of mice so thoroughly that "in spring of 1873 it was hard to 

 fmd a survivor". The wave of life was lost in the sand, and soon 

 things were as though nothing had happened. 



Pi..\NTS AND Animais.— Our object in this section is to analyse 

 and illustrate the ecological idea of the Balance of Nature; and we 

 naturally begin with the most fundamental relation, that between 

 green plants and animals. Those who have tried know the difficulty 

 of adjusting the balance of plants and animals in a self-contained 

 aquarium which is not artificially aerated. At one time the plants 

 get the upper hand and may crowd the water so that the animals 

 have no room to move about. At another time the animals get the 

 upjxT hand, and by devouring all the plants leave a water so poor 

 in oxygen and so abundant in carbon dioxide that they suffocate. 

 In other cases the animals are poisoned by their own nitrogenous 

 waste products, which arc normally absorbed and utilised by the 

 green j^lants. Now. the point is that these aquarium disasters are 

 very unusual in natural conditions. 



The most fundamentally important vital process in the world is 

 the photosynthesis effected by green plants. They utilise the energy 

 of tlie red-orange-yellow rays of the sunlight to build up carbon 

 dioxide and water into sugars and other carbon-compounds, at the 

 same time liberating oxygen as an all-important by-product. The 

 carbon-compounds made in the green leaves form the food of the 



