36 THE BIOLOGY OF THE SEASONS 



" Animate Nature " (for shortness) is conspicuously 

 economical. 



After the appearance of the hind-legs, the larvae come 

 often to the surface to breathe. They are learning to use 

 their lungs, which have been slowly developing for some 

 time as pockets projecting into the body-cavity from the 

 under side of the gullet. The tadpoles are now about 

 two months old, and in having lungs as well as gills they 

 may be compared to the double-breathing Mud-Fishes or 

 Dipnoi. As the lungs become established and functional, 

 the gills dwindle, and an intricate series of internal changes 

 leads from an essentially fish-like heart and circulation 

 to the characteristic Amphibian arrangements. 



After a period of hearty feeding, with consequent in- 

 crease in size and strength, the tadpole begins to show 

 signs of approaching metamorphosis. It loses its appetite, 

 it becomes much less energetic. The tail begins to break 

 up internally, its muscles and other structures become 

 disintegrated and dissolved, and most of the material 

 is swept away in the blood stream to help in building 

 up a better head. Wandering amoeboid cells, which are 

 present in almost all animals except threadworms and 

 lancelets, seem to play an important part in the extra- 

 ordinary process of absorbing the tail, working like sappers 

 and miners among the debris, dissolving some of the 

 material, carrying some away. In certain respects what 

 occurs is comparable to violent inflammation. It is like a 

 pathological process which has become normal, and thus 

 from watching tadpoles we get a glimpse of a deep-reaching 

 theory of disease as " a perturbation which contains no 

 elements essentially different from those of health, but 

 elements presented in a different and less useful order." 

 Often, at least, a disease implies a series of metabolic 

 changes which are not in themselves in any way extra- 



