180 



THE INFLUENCE OF INANIMATE SURROUNDINGS. 



some reservations ; for, in the first place, we know that various 

 animals are, as a rule, influenced in different ways by the 

 medium in which they live ; and besides, we cannot assert, on 

 the basis of any experimental research, that certain gases which 

 are injurious to men or to birds may not be indifferent or even 



FIG. 52. Tracheal system indicated within the outline a of a Water-bug, 6 of the larva of 

 an ^Eschna. The tracheae are shaded. 



advantageous to other animals. For instance, it is well known 

 that many larvae of insects live in situations, as in decaying 

 matter, where the air is undoubtedly mixed with gases which 

 tne higher "Vertebrata could not breathe without injury; also 

 that the capability for resisting the effects of iirespirable or 



