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the constitution of the central nervous system. The great majority of 

 insect actions can only be accounted for in this way, and once all 

 these actions were thus explained under the term instinctive. 



Secondly, we have those actions which are the result of sensational 

 principles, either the product of direct stimulus or of habits generated 

 from direct stimulus. 



On account of these latter principles and possibly others yet to 

 be discovered the mass of insect actions that can be truly called 

 instinctive, that is originating entirely within the brain, is ever decreasing 

 and may one day cease to exist. 



The third way in which insect actions can often be explained is 

 by reference to mechanical laws. Many attempts to give mechanical 

 explanations of biological phenomena have been made from time to 

 time ; as the contact-pressure theory of SCHWENDENER to explain the 

 arrangement and origin of the lateral members of a plant shoot ; the 

 mechanical embryology of His & Roux ; and in particular the explanation 

 of the segmentation of the ovum by HAECKEL, in which the various 

 forms are shown to result mechanically from the larger or smaller 

 quantity of yelk within the ovum. In the insect world perhaps the 

 most interesting example of the application of mechanical principles 

 is the explanation of the formation of hexagonal cells by the Bee. It 

 has been shown that not only is this form best under the given conditions 

 but that mechanical necessity compels the bees to construct six-sided 

 cells, no other alternative being possible. 



