70 Causes of Emigrations 



"This ability appears to be due to the presence of a very imper- 

 meable cuticle and a very efficient mechanism of hairs and bristles 

 protecting the spiracular openings" (Thorpe, 1931) . In the oil vats 

 in California, fly larvae subsist on organic food which consists 

 largely of the bodies of dead insects. In the vats their intestines are 

 full of crude oil, which, however, does not come in contact with 

 living cells. Adult ephydrid flies walk about on oil without being 

 injured. 



"A characteristic feature of living organisms is the possession of 

 mechanisms which protect them against the effects of changes of 

 their environments. 



"These mechanisms in their earlier forms exert their action by 

 restricting the interchange which they allow between the organism 

 and its surroundings. As they develop in efficiency, they become 

 more selective in action, and are able to preserve the essential char- 

 acters of the organism while allowing a free interchange with its 

 environment. They have preserved, even in the higher organisms, 

 some of the conditions of cell life which probably existed at very 

 early stages of their evolution. 



"As the complexity of organisms has increased, they have ren- 

 dered themselves more independent of their external environment by 

 providing their cells with an immediate environment of their own. 

 By this means external changes are only allowed to reach the cells in 

 a modified form. The possession of this internal environment en- 

 ables the organism to obtain the advantages of a freer interchange 

 with its surroundings without endangering the stability of its 

 essential living matter. 



"The evolutionary development of the adaptive mechanisms of 

 the organism has continually extended the range and scope of its 

 control over its environment. ... As the efl'ectiveness of the 

 mechanisms of the environment to its needs has increased, the need 

 for further adaptive modification of the organism has correspond- 

 ingly diminished" (Wardlaw, 1931). 



