540 



IRRATIONALITY OF INSTINCT. 



guided in its ordinary building operations by such an amount 

 of intelligence as would lead it to choose and execute its 

 various movements with a design to accomplish certain ends, 

 the same intelligence would direct it to leave these actions 

 unperformed when the purpose no longer required it ; instead 

 of which, we see that the animal is impelled by an internal 

 impulse to construct erections as nearly resembling those 

 which it would build-up on the banks of its native streams, 

 as the materials and circumstances will permit. Other ani- 

 mals are, in like manner, occasionally conducted by their 

 natural instincts to the performance of actions equally irra- 

 tional, and quite incapable of answering the purpose which 

 the particular instinct is destined to serve. Thus the Hen 

 will sit upon an egg-shaped piece of chalk, as readily as upon 

 her own egg, being deceived without difficulty by the general 

 similarity of its appearance ; and the Flesh-fly lays its eggs 

 in the petals of the carrion-flower, whose odour so much 

 resembles that of tainted meat as evidently to furnish the 

 same attraction to the insect. 



Fig. 281. NEST OF REPUBLICAN GKOSBEAK. 



710. Societies like those of the Beaver are rare among 

 Birds, whose associations are usually less perfect. There is 



