24 INTRODUCTORY. PLANTS AND ANIMALS 



which were produced early in the history of life, and 

 they have not become differentiated like the higher 

 organisms, the unmistakable familiar animals and 

 plants. Thus there is a minute oval organism called 

 Chlamydomonas (Fig. 20, p. 185) which lives in pools. 

 It is green and lives exclusively on liquid and gaseous 

 food like a plant, but during most of its life it swims 

 actively about like an aquatic animal. It must be 

 reckoned as a plant, because, as we saw, the mode of 

 feeding is the basal character of difference, but it still 

 retains the animal character of free locomotion. And 

 there are other minute organisms which feed in both 

 ways, partly on liquid and gaseous inorganic food, 

 partly on solid organic food, and these escape the 

 meshes of any definition of an animal or of a plant. 



We cannot, in fact, frame any comprehensive definition 

 which shall sharply separate all living organisms into 

 animals and plants. This conclusion illustrates a 

 truth that the student will realise more and more fully 

 as his biological studies progress, namely, that it is 

 useless to expect the facts of nature exactly to fit our 

 definitions, however carefully framed. There are so 

 many different ways in which living substances may 

 take shape, so many varieties of function, i.e. of ways 

 in which it may work, and so many possible combina- 

 tions of both of these, that there are certain to be excep- 

 tions to every rule we try to lay down. 'But that does 

 not mean that we have to give up the task of analysing 

 and classifying form and function, or the science of 

 biology would be impossible. We can always recognise 

 types of structure and types of function to which organisms 

 conform, more or less closely, because their physical 

 and chemical constitution forces them, so to speak, 

 along certain lines of differentiation and behaviour. 



