THE FROG in 



to be passed out again through a temporary or permanent aperture 

 for egestion, the anus. To co-ordinate these various parts a central 

 nervous system is developed, and in connection with the movements 

 a series of sense organs, which put the animal into touch with the 

 outside world. Practically all the striking characters of an animal 

 are concerned with the question of food, and its whole structure is a 

 complex combination of parts, enabling it to obtain its food readily. 

 In many cases this main object is modified, and sometimes to a large 

 extent by another aim, and that is to enable the animal itself to 

 escape being utilised as food by another animal. All these structural 

 modifications that are for some useful purpose we term adaptations. 

 One further striking difference between animals and plants 

 remains to be noticed, and that is, their gaseous exchanges with the 

 atmosphere. In the case of all organisms, save certain lowly plants, 

 respiration, i.e. the exchange of carbon dioxide for the oxygen of 

 the air occurs, and it is more rapid in animals than in plants. Those 

 plants that possess chlorophyll or an allied substance, in the presence 

 of sunlight, take in from the air carbon dioxide, from which they 

 remove the carbon, setting free the oxygen into the air again. This 

 second exchange, which does not occur in animals, is by far the 

 larger of the two in green plants, and almost completely masks the 

 other. 



All the vital manifestations of both animals and plants are in 

 reality manifestations of energy, and so it will be well to return 

 to the second phenomenon characteristic of living beings, and 

 examine quite generally the way in which energy is obtained, stored 

 and transformed by organisms. Two kinds of energy are dis- 

 tinguishable : one is kinetic energy, that is, energy that is mani- 

 fested in the form of motion, heat, light or chemical or electrical 

 changes ; the other, termed potential energy, is energy that is 

 stored up in a quiescent condition, only needing some stimulus to 

 release it and allow it to become transformed into kinetic energy. 

 Potential energy in the living being is stored up in a series of fairly 

 complex chemical compounds. Such energy is stored as the result 

 of a complicated sequence of chemical reactions, which lead to the 

 formation of compounds of higher and higher chemical complexity, 

 until we reach that highly organised substance, or intimate mixture 

 of substances, which we term protoplasm. We use the term 

 Anabolism to include all these constructive changes culminating in 

 the building up of protoplasm, and this represents, as it were, the 

 credit side of the account, the storage of energy. On the other side 

 we have the debit account, the expenditure of this reserve in the 

 form of kinetic energy brought about by the breaking down of the 



