THE LIVING WORLD 247 



ascend to 18,000 feet in the Andes. The viper is found 

 in the Alps at 5000 feet above the sea, and in South 

 America the condor will soar to more than 22,000 feet 

 above the sea-level. 



The ocean has different inhabitants at different depths, 

 and there seems to be no depth-limit to life, especially 

 to animal life. At a depth of 2000 fathoms the ocean 

 fauna present much richness and variety. 



As to the inter-relations of living organisms, one 

 great organic inter-relation, already noticed, underlies all 

 others namely that by which oxygen is set free by 

 plants to be made use of in respiration and so recombined 

 with carbon a process which has been called the " cir- 

 culation of the elements." 



The phenomena of parasitism constitute a very common 

 relation between organisms, parasites being generally 

 more or less inimical to their hosts. 



Besides the evident relation of " enemies " and " rivals," 

 organisms may also directly or indirectly benefit other 

 creatures. 



Thus ants and the bull's-horn acacia benefit each 

 other directly. The plant furnishes food and lodging 

 (in special cavities) to the insects, which in turn are 

 ready to rush out and bite furiously any animals 

 which attempt to injure the tree. Organisms may be 

 benefited indirectly by others which destroy the enemies 

 or rivals of the former. Again, services may be rendered 

 in very curious ways. Thus barnacles, which fix them- 

 selves on whales, are provided with an extra amount of 

 food by being so carried about. A small fish has also 

 been found to live within the interior of a sea-anemone, 

 feeding on portions of the latter's food. Birds dissemi- 

 nate seeds which they have swallowed, and sometimes 

 do so by carrying them or the eggs of small animals 



