CH. XIX Influence of Habits and Surroundings 309 



dwelling. It is difficult, however, to abstract the influence 

 of restricted space from associated abnormal conditions. 



{fi) Chemical Influences. — Quieter, but more potent, are 

 the chemical influences which damp or fan the fire of life, 

 which corrode the skin or drug the system, which fatten or 

 starve, depress or stimulate. Along with these we must 

 include that most important factor — food. 



When a lighted piece of tinder is placed in a vessel 

 full of oxygen it bums more actively. Similarly, super- 

 abundance of oxygen makes insects jump, makes the 

 simplest animals more agile, and causes the "phosphores- 

 cent " lights of luminous insects to glow more brightly ; 

 and young creatures usually develop more or less rapidly 

 according as the aeration is abundant or deficient. The 

 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. 



By keeping tadpoles in unnatural conditions the usual 

 duration of the gilled stage may be prolonged for two or three 

 years. The well-known story of the Axolotl and the Ambly- 

 stoma is suggestive but not convincing of the influence of 

 surroundings. These two newt-like Amphibians differ slightly 

 from one another, in this especially that the Axolotl retains 

 its gills after it has developed lungs, while the Ambfysioma 

 loses them. Both forms may reproduce, and they were 

 originally referred to different genera. But some Axolotls 

 which had been kept with scant water in the Jardin des 

 Plantes in Paris turned into the Amblystoma form ; the two 

 forms are different phases of the same animal. It was a 

 natural inference that the Axolotls were those which had 

 remained or had been kept in the water, the Amblystoma 

 forms were those which got ashore. But both kinds may 

 be found in the water of the same lake and the metamor- 

 phosis may take place in the water as well as on the shore. 

 For these and for other reasons this oft -told tale is not 



