58 THE BUTTERFLY VIVARIUM. 



shell at the destined time, regardless of surrounding 

 cold or heat. Many experiments have heen made 

 with the view to accelerate the hatching of insect 

 eggs hy the stimulus of heat, and to retard them by 

 the application of intense cold ; but, except in a very 

 few cases, little or no effect was produced— period- 

 icity, rather than any kind of atmospheric in- 

 fluence, being the governing power which regulates 

 the hatching time. In some few instances, however, 

 as stated, the time can be accelerated by warmth — 

 as with Silk-worms, for example— which is, perhaps, 

 owing to their existence in Europe being altoge- 

 ther artificial, and their instincts being more or less 

 thwarted and confused in all their stages. It has been 

 found much more difficult, and in many cases im- 

 possible, to retard the period of hatching by any 

 degree of cold ; and certain eggs destined to hatch 

 in June, for instance, will, according to Brahm, 

 hatch at that time even in an ice-house. 



Eggs will, in fact, bear an extraordinary degree 

 of either heat or cold without injury, and without 

 being in the slightest degree influenced by its appli- 

 cation, but the absence of air destroys their vitality 

 very quickly. Spalanchini found that the vital 

 principle was destroyed in all those which he sub- 

 mitted to the effect of perfect vacuum in an air-pump. 



