xx THE INFLUENCE OF ENVIRONMENT 389 



most active animals birds and insects live in the air 

 and have much air in their bodies ; sluggish animals 

 often live where oxygen is scarce ; changes in the quality 

 of the atmosphere may have been of importance in the 

 historical evolution of animals. Fresh air influences the 

 pitch of human life, and lung diseases increase in direct 

 ratio to the amount of crowded indoor labour in an area. 

 It is obvious that in part this simply means that the 



FIG. 119. AXOLOTL(INTHE WATER) AND AMBLYSTOMA (ON THE LAND). 



opportunities for infection with the tubercle bacillus 

 are greatly increased by crowding, but there is a positive 

 tonic value in fresh air. 



By keeping tadpoles in sub-aquatic conditions the usual 

 duration of the gilled stage may be prolonged for two cr 

 three years, and giants arc thus produced. The well- 

 known story of the Axolotl and the Amblystoma may 

 be recalled. These two newt-like Amphibians differ 

 slightly in shape, and in this, that the Axolotl retains 



