34 LECTUEES ON BIOLOGY 



parts of oxygen. If, however, this percentage is lowered to ten, 

 we observe in man and the higher mammals serious difficulties 

 of breathing. A further decrease down to seven per cent, 

 inevitably produces death by suffocation. 



There is a wide divergence in the behaviour under equal con- 

 ditions of warm-blooded and cold-blooded animals. If we place 

 a rabbit in a room containing an insufficient percentage of oxygen 

 it will after a few minutes have convulsions and die ; frogs, how- 

 ever, will remain alive for many hours, even in an atmosphere 

 from which all oxygen has been removed. These facts are 

 obviously connected with the varying consumption of oxygen 

 by the various species. While, for instance, the small songsters 

 consume 12 grammes of oxygen per kilogramme of body-weight 

 per hour, the rabbit consumes 1 gramme, man about 0*5, and the 

 frog only 0'07. 



Just as we are able to decrease the supply of oxygen without 

 a resultant disturbance of the functions of life, so we are similarly 

 able to increase it. Thus mammals are able under ordinary 

 pressure to live in pure oxygen. If, however, the pressure is 

 increased to three atmospheres, death results, curiously enough, 

 from suffocation. In ordinary air, however, animals can support 

 a pressure up to fifteen atmospheres. The researches by Bert 

 show, further, that a low percentage of oxygen may be balanced 

 by an increase in air-pressure, whilst, conversely, the deleterious 

 effects of high pressure may be balanced by a decrease in the 

 supply of oxygen. This operates, however, only within certain 

 very narrow limits. 



The power of resistance of aquatic animals to water-pressure 

 is remarkable. The results of deep-sea expeditions undertaken 

 by various Governments have clearly shown that the greatest 

 depths of ocean, abysses of 6,000 to 8,000 metres, are inhabited 

 by organisms able to withstand a pressure of several hundred 

 atmospheres. And not only do we find at these depths primitive 

 forms, but also representatives of the higher species, cuttle- 

 fishes, crustaceans, and even fishes. (See coloured plate, No. 2.) 

 Plants are found at a depth of about 4,000 metres, but beyond 

 that depth existence becomes to them impossible, owing to the 

 total absence of light. 



If fishes from such depths are rapidly brought to the surface 



