THE FROG 109 



of various waste products, which have to be removed from the 

 body by excretion. As has been pointed out previously, while the 

 building up processes are in the ascendant the organism increases in 

 size, i.e. it exhibits the power of growth. Again, all living beings 

 obtain energy from the oxidation of their tissues, and this energy 

 appears in the form of heat motion and electrical changes. These 

 particular alterations are more striking in animals than in plants, 

 but, nevertheless, are common to all living beings, which are there- 

 fore capable of bringing about a transformation of energy, a point 

 with which we shall deal more fully later. 



Thirdly, plants and animals have the power of replying to 

 messages from their environment. This response to external 

 stimuli is not, as a rule, vague or local, but definite and general, and 

 we say, therefore, that they possess irritability or sensitivity. 



Lastly, we have just seen, in the case of the frog in particular, 

 that all living beings pass through a definite life-cycle. They 

 commence as fertilised ova, or as an actual part of the parent, and 

 go through a certain definite pre-ordained series of changes, as the 

 result of which they become adult. When adult they possess the 

 power of carrying on the race by reproducing their like, and, after 

 a certain period in the possession of this power, long or short, as 

 the case may be, they pass on into old age or senescence. They 

 finally die, for death is also an integral part of the life-cycle. 



Thus far we have been considering the properties common to 

 living beings, and although we are here concerned with Zoology, it 

 will not be out of place to consider briefly the main differences 

 between animals and plants. The differences between the higher 

 animals, such as the frog, dog, or cat, and the higher plants, such as 

 shrubs and trees, are so great that they are apparent to the most 

 superficial observer, and need not be dealt with here. In the case 

 of the lowest members of both kingdoms, simple animals and simple 

 plants composed in many cases of but a single cell, the differences 

 become less and less obvious. Indeed, we actually find a number 

 of organisms, including, for example, the slime fungi, found growing 

 in tan pits and on decaying wood, are claimed by both zoologist and 

 botanist. It has been proposed to give the name Protista to this 

 group of beings, which are a mixture of animal and plant, or live 

 as an animal or plant according to the conditions of their environ- 

 ment. It is probable that ah* living beings have descended from 

 some such primitive organism possessing the potentiality of becoming 

 either animal or plant, and all exhibiting to some degree the various 

 vital powers we have just discussed. 



The most fundamental difference between animals and plants 

 is in the chemical constitution of the protoplasm which, small though 



