180 THE INFLUENCE OF INANIMATE SURE0UNDJN6S. 



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 



Fio. 52.— Tracheal system indicated within the outline a of a Watev-bng, b of the larva of 

 an jEschna. The traoheas are shaded. 



advantageous to othfer animals. Tor instance, it is well known 

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

 matter, where the air is undoubtedly mixed with gases which 

 tne higher Yertebrata could not breathe without injury also 

 that the capability for resisting the effects of irrespirable or 



