OXYGEN IN AIE AND WATER. 195 



aquatic animal whose organs of respiration are adapted to 

 breatte water should be capable of learning to breathe air with 

 them. If, however, we enquire somewhat more closely into the 

 character of the process, it loses much of its strangeness. In 

 both cases the oxygen is absorbed from the surrounding 

 medium by a membrane which is kept moist, and to which it 

 must be a matter of indifference whether it receives it from 

 air or from water. Thus, granting that in both cases the 

 osmotic power of the respiratory skin remains the same; as 

 the amount of' oxygen taken up within a given time naturally 

 depends on the proportion of oxygen contained in equal deter- 

 mined volumes of the air or of the water, the respiratory 

 surface may be in a position to take up more oxygen from 

 the air than from the water in the same unit of time, be- 

 cause air has a larger admixture of free oxygen. Thus — if there 

 is no other hindrance to an alteration in the mode of life — on 

 the above hypothesis, an animal, which has hitherto breathed 

 in water, will more easily accustom itself to breathe in air 

 than an animal living in the air, on the contrary, can accom- 

 modate itself to breathing in the water ; for in this latter case 

 a deficiency, which must inevitably arise, must first be covered 

 by auxiliary organs — by the skin, for instance — ^while in the 

 former the originally small requirements of the water-breathing 

 animal will be much more easily suppKed by accommodation to 

 the more copious respiration of air than by its continuiag to 

 breathe in water. But whether the creature is, without excep- 

 tion, benefited by a change of function, by which a medium 

 poor in oxygen is exchanged for one rich in that element, of 

 course is not proved ; while, on the other hand, it cannot be 

 disputed that some such advantage may be coimected with it.'* 

 The causes which prompt an animal to quit the water and 

 to accustom itself to breathe air may be of very various natures ; 

 lack of food, need of shelter, and the pursuit of prey or flight 

 from foes most likely play the most important part. It is, 

 however, also possible that other causes which are less obvious 

 may have led to the same result; thus, for instance, the 

 absolute deficiency of air in the water under some circum- 

 stances may undoubtedly have exerted this influence. When 



