EXTERNAL INFLUENCES AS CAUSES OF VARIATION 



233 



It is both something less and something more. It is less by so 

 much as the individual has failed to develop because of unfavor- 

 able external conditions ; it is more by whatever development is 

 due to the direct influence of external conditions. 



This is the principal reason why breeders have difficulty in 

 knowing how much to credit to inheritance, and it is on this 

 account that more knowledge is needed of the nature and extent 

 of the development due directly to external causes, and therefore 

 independent of inheritance. After all, however, it will be found 

 that the total development from all causes is well within hereditary 

 limits except, perhaps, in rare cases where the normal functions 

 may have been altered by unusual conditions. 



SECTION IV EFFECT OF CONTACT UPON PROTOPLASMIC 



ACTIVITY * 



Unstable chemical compounds are exceedingly sensitive to 

 mechanical contact either from solid bodies or from liquids or 

 gases in rapid motion. Davenport says : 2 



Mechanical disturbance can induce in certain lifeless compounds violent 

 chemical changes. Compounds which are so affected are preeminently 

 unstable. This instability, however, varies greatly in degree. In some 

 cases the blow of a hammer is required to upset the molecules, the result 

 being often a violent explosion. In other cases (e.g. chloride or iodide of 

 nitrogen 3 ) the slightest touch of a feather suffices to produce an explosion. 

 Now, most of the substances which explode upon impact [altering their 

 chemical arrangement and properties] . . . are organic compounds, 

 fulminate, nitroglycerin, gun cotton, and picric-acid derivatives, and 

 therefore it is not surprising that we find the notoriously unstable proto- 

 plasm violently affected by contact. 



" Of special interest in this connection " is the fact that 

 "periodic disturbances produce very important molecular changes 

 in [certain] chemical compounds. Certain substances have a 



1 C. B. Davenport, Experimental Morphology, Part I, pp. 97-110, and Part II, 

 PP- 370-388, from which most of the data on this subject are taken. N. B. Con- 

 tact agents are technically known as molar agents. 



2 Ibid. Part I, pp. 97-98. 



3 When chemical terms of this kind are used outside of quotations the new 

 form will be used, as nitrogen chlorid, omitting the e. 



