^"^ AND ^"^/i^ 



JOURNAL OF VARIATION. 



Vol. VII. No. 1. September 15th, 1895. 



The Resting Habit of Insects as Exliibited in the Phenomena of 

 Hybernation and Aestivation.-' 



By J. W. TUTT, F.E.S. 



The old notion that the evohition of the organic world was brought 

 about by the action of internal forces appears to have almost entirely given 

 place, in the minds of modern philosophers, to the belief that the lines 

 on which evolution proceeds are entirely determined by external influ- 

 ences, and that, far from such influences having played but a secondary 

 part in the process of evolution (in the direction of improving and 

 modifying the results produced by the internal forces), the evolution of 

 organic beings is due to the action of these external forces upon the 

 organism itself. The older view ascribed, as it were, to organisms a 

 motive force ; the view taken now is that the internal forces, so far as 

 they exist, are only made active by their reaction to the external forces 

 which surround the organism. 



In his Effect of External Influences upon Development, Professor 

 Weismann illustrates the potency of external influences by various 

 examples, and shows how the vital manifestations of animals and 

 plants are ultimately reactions to such influences. He compares 

 animals and plants to "machines, so constructed that stimuli from the 

 outer world cause them to act in the most purposeful manner for their 

 own maintenance." The adaptations, which are the result of such 

 action and reaction, exist everywhere among organic beings to a very 

 high degree, although it follows of necessity that such adaptations 

 must be considered as relatively rather than absolutely perfect. This 

 power of adaptation proves that the internal forces of the organism 

 have helped to bring about the desired result, but only in response to 

 external stimuli, which determine the lines on which the internal 

 forces shall act. If this be admitted, and it is difficult to see how it 

 can be denied, we acknowledge the efficiency of external causes to bring- 

 about evolution ; but in Avhat way the external stimuli have acted on 

 the organism, and in what way the internal forces have reacted in 

 response to the external stimuli so as to bring about the changes 

 necessary in the formation and transformation of organisms is not at 



* Some notes suggested by The Romanes Lectm-e of 1894, entitled, " The Effect 

 of External Influences upon Development," 



