392 CONCLUSION 



belief is obtaining more and more detailed support 

 from recent research into the biochemistry of different 

 species (p. 43). 



The differences between closely allied but distinct 

 kinds of animals and plants probably depend mainly 

 on comparatively small differences in some of the 

 proteins of which their protoplasm is composed, or on 

 differences in the forms of aggregation of their protein 

 molecules. When life first appeared upon the earth, 

 or rather in the water, there were probably different 

 kinds of very simple organisms having such differ- 

 ences, which affected their behaviour and consequently 

 their mode of life. As to the details of such differences 

 we know of course nothing, but it is evident that they 

 must have gradually led to the adoption of such very 

 different modes of life as we see to-day distinguishing, 

 for instance, the animals from the plants. There are 

 still existing many different kinds of minute unicellular 

 organisms (Protista) which are neither unmistakable 

 animals nor unmistakable plants, but which show a 

 mixture of animal and plant characters. Yet the vast 

 majority of organisms are either distinctively animals 

 or distinctively plants. 



The main characteristic of animals — on which all 

 their other features are based — ^is that they can only 

 take the nitrogen which all organisms must take in 

 to make their proteins from ready made proteins, and 

 since proteins are only found in nature in the bodies 

 or as products of the bodies of organisms, animals must 

 consequently feed on other organisms or on organic 

 products. Plants, on the other hand, can take their 

 nitrogen in simpler forms — as simple salts, or even in 

 certain cases as free nitrogen. The colourless plants, 

 which in the matter of nutrition are intermediate 



