434 EVOLUTION AND GENETICS 



which we can explain in part. These forces ultimately resolve 

 themselves into matter and energy operating in space and time, 

 and here we must be contented to stop. 



The Tetrakinetic Theory. Osborn expressed views similar to 

 these more than a decade ago. Under the name of the tetrakinetic 

 theory he classified the factors in life as four energy complexes, 

 (1) the inorganic environment, (2) the life environment, (3) the 

 organism, and (4) the heredity-germ. In spite of his masterly 

 analysis the literature of evolution has unfortunately continued 

 along the same old lines. 



The interpretation is equivalent to sa>ang that all organic 

 change is based upon changes in the complex environment, refer- 

 able ultimately to the four entities, matter and energy, space and 

 time. The position is logically tenable, but to satisfy the demands 

 of evolution we must give attention to other details. 



Adaptive and Incidental Change. There is every reason to 

 beheve that variation of the environment is the immediate stimulus 

 which induces variation in organisms. This does not imply that 

 the environment actually shapes the organism, but it determines 

 the expression lohich the organic heritage will attain. Even identical 

 twins are never really identical. They are genetically the same 

 because they are derived from halves of the same fertilized ovum, 

 but two indi\'iduals can never be surrounded by exactly the same 

 conditions. They cannot eat exactly the same food nor the same 

 quantity; they cannot sit in the same chair at the same time; the 

 minute differences in their contacts may even mean life or death, 

 as is true of spatial relations in the traffic of city streets at any 

 time. Such differences as they display must therefore be due to 

 the reaction of their identical heritages to different stimuli. 



The suggestion that change in organisms arises in response to 

 change in the environment docs not mean that a stimulus always 

 brings about a response which fits the organism to meet the 

 existing environmental condition; in other words adaptations do 

 not necessarily arise as a response to the environmental condition 

 which they enable the organism to meet. Adaptive changes may 

 appear in this way, as when the rays of the sun stimulate the 

 l^ody to deposit pigment for protection, but changes may occur 

 as incidental results. Rickets, although due to lack of sunhght 

 in some cases, unfits an individual for normal life, and the normal 

 skeletal development which is in part due to abundance of sunlight 



