Some Essential Facts and Two Fundamental Questions

The classical definition differentiates hormones from a variety of other substances which also act in mammals in minute concentration to produce profound biological effects, such as (1) vitamins, chemical regulators contained in the diet, (2) certain drugs, (3) humoral agents which exert a 

Each hormone produces its characteristic action in diverse species. 

As a general rule, although there are some rare exceptions, the hormone from one species is active in all other species of mammals. The characteristic over-all responses produced by the individual hormones and the principal target cells which they affect have been adequately described in standard works (Pincus and Thimann, 1948 1950 Selye, 1949) and need not be repeated here. 

From these essentials, two basic questions have attracted major interest in previous attempts to construct a theory in explanation of the modes of hormone action  

How can trace amounts of hormone produce profound biological effects in target cells without contributing either significant amounts of energy or matter to the system  

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