SYMBIONTICISM AND ORGANIC EVOLUTION 139 



advanced in the past, attempts have been made to explain 

 numerous specific variations in plants and particularly in 

 animals. These explanations have not been favorably 

 received among biologists. It is not necessary to review 

 this extensive literature, as the well informed biologist is 

 acquainted with the analyses and the criticisms that have 

 been launched against them. The general reader who 

 desires to learn more about these views and criticisms is 

 referred to Kellog's "Darwinism Today" ('07), which 

 gives a comprehensive and unbiased discussion of the 

 subject. 



In the evolution of higher organisms, the modifications 

 of structure and function is ever increasing so that in the 

 mammaha and in man the organism has become an extremely 

 complex mechanism. The normal activity of a single 

 tissue or organ is not only dependent upon its particular 

 intrinsic organization, but as recently demonstrated, upon 

 a number of extrinsic factors as well. Thus, it has come 

 to be recognized in endocrinology that few, if any, of the 

 organs of internal secretion perform their function indepen- 

 dent of other organs in the system. The inter-relationship 

 of various activities is well illustrated in the mechanism 

 of carbohydrate utilization in the mammahan body. 

 Starches are digested in the intestinal tract under the 

 influence of digestive enzymes produced by gland cells. 

 (The importance of symbiotic bacteria in the digestion of 

 cellulose by ruminants was previously mentioned.) The 

 cells of the body, unassisted, are not able to utihze the sugar 

 produced in this digestion. It has been known for many 

 years that a substance, a hormone, is produced in special 

 cells located in the islands of Langerhans of the pancreas 

 that is essential for sugar metabolism by the cells of the 

 animal body. This hormone has been named "Insulin." 

 Banting and Best ('22) succeeded in isolating this chemical 

 substance and more recently Abel ('26) has obtained it in 



