80 De Vi Physica 



entiation a constant passage from potential 

 to actual, synthetic to evolved, atmospheric 

 conditions : from a state relatively thick, 

 mephitic, hot, miasmatic, pestilential, dark, 

 to one relatively purer and ever purer, 

 drier, cooler, lighter, and more various. The 

 * lower ' any animal is, the more ' poisonous ' 

 the conditions under which it can live. If 

 a reptile, a bird, and a mammal are shut 

 up in a close chamber with but little air, 

 the bird will be the first, and the reptile 

 the last, to die. And if tadpoles are kept 

 without light, they will never become frogs. 

 Such facts as these give us, as it were, 

 glimpses of the past history of the world. 

 And when we combine with this much 

 evidence that shows how Nature instantly 

 responds to the call of circumstances, how 

 animals change and mysteriously assume 

 colour or coating suited to new localities : 

 the conclusion almost seems necessary, that 



