INTRODUCTORY PROBLEMS 17 



dead animal rots away, that is, it is reduced by bacteria to 

 relatively simple salts and gases which may form the food of 

 plants, (c) Green plants are able, with the help of the sunlight, 

 to utilise the carbon dioxide which is given off as a waste gas 

 from animals. (This waste gas is also necessarily produced by 

 plants, but this is disguised during the day by the peculiar nutritive 

 process, the first step in which is the reduction of carbon dioxide. 

 The liberation of carbon dioxide by plants must be studied in 

 parts which are not green, e.g. seeds, or at night, when the 

 nutritive process has stopped.) (d) There is an inter-relation 

 of incalculable practical importance between insects and flowers, 

 since the insect visitors secure pollination, (e) There are scores 

 of other familiar inter-relations of great interest, such as those 

 implied in the diet of carnivorous plants, in the formation of 

 galls, in the remarkable mutually beneficial partnership between 

 certain unicellular algae and certain animals (Symbiosis'). 



The study of the inter-relations among animals is not less 

 fascinating. We may refer to the occurrence of quaint partner- 

 ships, as of crocodile and crocodile-bird ; the closer " commen- 

 salism " illustrated by certain hermit-crabs and their companion 

 sea-anemones ; the frequent occurrence of parasitism ; such 

 quaint phenomena as are presented by the existence of " guests," 

 " pets," and even " domestic animals " among ants ; the estab- 

 lishment of complex domestic and social relations, and so on. 



It may be said that of all the general ideas that it is useful for 

 the teacher to be at home with in relation to nature study, there 

 is none so fundamental as this idea of the web of life, the working 

 out of which was one of the most important of Darwin's con- 

 tributions to science. It comes to this, that no plant or animal 

 lives or dies to itself. 



BIBLIOGRAPHY. Gamble's Animal Life (London, 1908) ; Thomson's Study of Anitnal 

 Life (John Murray, London) ; and Science of Life (Blackie & Son, Glasgow). 



VOL. I. 2 



