2 



The Medium 



The first of the physical features of the environment to be considered 

 will be the medium— that is, the material which immediately surrounds 

 the organism and with which it has its all-important exchange. At 

 first sight one might think that many diverse media exist. Some 

 organisms live in the soil, and some in ponds; some thrive in manure 

 piles, and others enjoy a successful existence in the blood stream of 

 vertebrate animals. Certain nematodes live in vinegar, and a fly 

 larva of the genus Psilopa grows in petroleum. Once during a de- 

 partmental gathering at Cambridge University a member of the staff 

 entered the room waving a journal in which the habits of this larva 

 were reported. "Look here," he said, "in this report an insect is 

 described which lives in petroleum. The first thing you know it will 

 parasitize our motor cars!" 



The medium in each of the above examples, and indeed that for 

 organisms in every natural situation, is either a liquid or a gas, and it 

 is usually air or water. Although animals and plants inhabiting soil 

 or mud may at first appear to be exceptions, a closer scrutiny shows 

 that a film of air or water around each organism is actually the ma- 

 terial in immediate contact with it. An enlarged view of the small 

 animals living in the wet sand of the seashore shows that their essen- 

 tial exchange is with the water percolating between the sand grains 

 and that the medium for these animals is sea water, not sand (Fig. 

 2.1). The term medium is thus used in a strict sense and is distin- 

 guished from the substratum, or surface on or in which the organism 

 lives. 



The existence of air and water as the fundamental media divides 

 the world into two major environments: terrestrial and aquatic. The 

 media are not completely isolated from each other, however; some of 

 the atmospheric gases are dissolved in all natural waters, and some 

 moisture is present almost everywhere in the atmosphere. Differences 



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