THE ORGANISM AND ITS ENVIRONMENT 363 



glands of the Anopheles mosquito and the yellow fever micro- 

 organism (a filterable virus) in the glands of the iEdes mosquito. 

 These micro-organisms are injected into the host with the products 

 of the glands. Not all mosquitoes that are capable of harboring these 

 Protozoa are infected. Anopheles and ^des mosquitoes abound in 

 some localities that are free from malaria and yellow fever, A high 

 degree of success in eradicating these diseases has been attained in 

 certain regions, notably in the Panama Canal Zone, by the screening 

 of infected persons, and the elimination of mosquitoes by draining 

 stagnant waters and the free use of oil. A surface film of oil on the 

 breeding places shuts the larvae and pups ofT from air. 



The amazing diversity of type among insects accounts for their 

 almost universal presence, and their ability to live under the most 

 unexpected circumstances. One form passes its larval stage in pe- 

 troleum pools; another is able to bore its way through the lead 

 housings of electrical conduits in order to get at the insulation. The 

 drug store beetle lives on herbs and drugs that would cause the 

 death of other animals. While chance mutations are responsible for 

 the diversity of type, the adaption of certain species to a new food 

 supply must be regarded as primarily a non-genetic functional 

 adaptation, but the ability of the insects to adapt themselves to new 

 and different foods is to be regarded as a genetic character. 



Inheritable Functional Characters. Inheritable species char- 

 acters that are functional adaptations include all the diverse species 

 characteristics which enable the organism to utilize the sources of 

 food and energy in its environment to provide for perpetuation of 

 the species, and to seek surroundings suitable for its continued exist- 

 ence. Species functional adaptations are always associated with cor- 

 responding structural adaptations. These are particularly extensive 

 in the life cycles of parasitic animals. Obligatory parasites are in 

 many cases so sharply adapted to the tissues of certain hosts that 

 they are unable to subsist on any other source of food. The hook 

 worm, a member of the phylum Nemathelminthes, exists in its 



