94 THE CONTROL OF LIFE 



is satisfactory except that the surface does not give the 

 animals sufficient room for exercise. This is surely a 

 parable for our instruction. 



The French have a wise proverb, " By force of striking 

 one becomes a blacksmith " (C'est a force de forger qu^on 

 devient forgeron) ; and this is equally true of the powerful 

 wrist of the violinist. The results of physical exercises 

 show that the size and strength of a muscle may be 

 greatly increased by persistent exercise. It seems that 

 the muscle-fibres grow thicker and stronger ; we believe 

 we are right in saying that they do not become more 

 numerous. 



To a certain extent the colour of some birds, e.g. 

 canaries, can be altered by particular kinds of food. 

 In gulls and poultry the character of the food-canal 

 may change considerably in the individual bird accord- 

 ing to the nature of the food. The fattening of geese 

 is a somewhat passive nutritional modification. Mr. 

 Beebe's exceedingly interesting experiments have shown 

 that some birds, such as the bobolink, may be dieted so 

 that they keep their breeding plumage all the year and 

 will sing their spring song in midwinter. 



Extreme tanning or browning of the skin as the result 

 of many years of exposure to a tropical sun is a modifica- 

 tion that may persist for many years after the traveller 

 has returned to the temperate home country. It is 

 hardly correct to include the blanching of the banked- 

 up celery as a modification, for it does not last after the 

 peculiar conditions of darkness are removed. Some 

 caterpillars subjected to cold develop into dark-coloured 



