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 or RepuspLican GROSBEAK. 
710. Societies like those of the Beaver are rare among 
Birds, whose associations are usually less perfect. There is 
