2 EVOLUTION OF LIFE i. i- 



different kinds, while descendants of most unfish-like form are found 

 living out of the water and even in the air and under the ground ? 



To put it in a way more familiar, though perhaps less clear: what 

 are the forces that have produced the changes of animal form ? Know- 

 ing these forces, and the original type, it would be possible to con- 

 struct a truly general science of zoology, with sure premisses and 

 deductions. Even if we cannot reach this end, we should at least try, 

 hoping that after investigation of the biology of vertebrates it will be 

 possible to retain something more than a mass of detailed information. 

 At the end of such a study, if we deal with the subject right, we should 

 surely be better able to answer some of the fundamental biological 

 questions. We should be able to say something about the nature of 

 evolution and of the differences between types, to know whether there 

 have been rhythms of change at work to produce these differences, 

 and also — the acid test of any true science — to forecast how these 

 changes are likely to proceed in the future. 



2. What do we mean by the life of an animal? 



In biology we make much use of analogies, attempting to grasp the 

 nature of the processes at work by comparison with man-made 

 machines. We have a science of anatomy, which we are told is con- 

 cerned with the 'structure' of animals, and we feel that we understand 

 what 'structure' means. Physiology is the study of 'function', and 

 this, too, we seem to understand. We take the analogies from our 

 machines, which have what we call 'structure' and 'function'. How- 

 ever, the difficulty at once arises that the living things make and control 

 themselves. The whole scheme fails us when we ask what is it, then, 

 that we call the 'life' of the animal, and what is it that is passed on 

 from generation to generation, and that changes through the ages by 

 the process we call evolution ? It has gradually become apparent that 

 the body is not a fixed, definite 'structure' as it appears to casual 

 observation or when dissected. In life there is ceaseless activity and 

 change going on within the apparently constant framework of the body. 

 The movement of the blood is one sign of this activity, and since 

 Harvey's discovery of the circulation ( 1 628) we have learnt of innumer- 

 able others. Everyone knows that the skin is continually being renewed 

 by growth from below, and many other types of cell are similarly 

 replaced; for instance, red blood-cells last only for a few weeks in 

 man. Even in the cells that are not completely destroyed and replaced, 

 such as the nerve-cells, there is continual change of the molecules that 

 make up their substance. The full extent of this exchange has been 



