There is no subject of human study which may not be at some time or other of economic significance, and anything which affects the character, the ideals or the environment of man may make it necessary to modify our assumptions and our reasoning with regard to his conduct in economic affairs.
In the mechanical processes which we can experimentally modify at will, and which therefore we learn to apprehend with greatest fulness, whenever an effect on a body, B, is in causal connexion with a process instituted in another body, A, it is usually possible to discover a mechanical connexion between the two bodies which allows the influence of A to be traced all the way across the intervening region.
The relief of the land and varying degrees of rainfall and vegetation, however, serve to modify these conditions in many important particulars.
Various agencies are at work tending to modify the composition of the atmosphere, but these so neutralize each other as to leave it practically unaltered.
The next step was to endeavour so to modify and weaken the virus as to enable it to be used as a preventive or as an antitoxin.