300 THE HARMONIES OF NATURE. 



The cavity of the gizzard being necessarily very small, a crop 

 is as essential an appendage to this structure as the e hopper ' to 

 the mill ; it receives the food as it is swallowed, and supplies it 

 to the gizzard in small successive quantities as it is wanted. 

 To assist the triturating power of the gizzard, the birds have 

 been taught by an admirable instinct to swallow hard foreign 

 bodies, such as sand, gravel, or pebbles. Fowls grow lean if 

 deprived of stones, and no wonder, since experiment shows 

 that, unless the grains of corn are bruised and deprived of 

 their vitality, the gastric juice will not act upon or dissolve 

 them. 



Birds necessarily require an uncommon strength of vision, 

 both for discovering their prey and avoiding their enemies ; 

 for their prey is frequently small, and not easily to be dis- 

 tinguished from the surrounding objects, or is itself engaged 

 in rapid motion, so that often even the greatest velocity of 

 flight would have been unavailing, without the assistance of a 

 piercing eye. A bird hovering in the air can be seen from a 

 vast distance by a sharp-sighted enemy, and thus also needs a 

 keen vision, to be able to escape in due time from the impend- 

 ing danger. 



Besides the faculty of embracing a vast field of vision is the 

 essential adjunct of considerable powers of locomotion ; for it is 

 evident that short-sighted and at the same time swiftly-flying 

 birds must soon have perished from this want of harmony 

 in their structure, their obtuse vision being a constant im- 

 pediment to the full exercise of their vigorous wings. 



For all these reasons the birds have been gifted with a sharp- 

 ness of vision vastly superior to that of the quadrupeds. A 

 sparrow will detect a grain of corn at the distance of eighty 

 feet ; a hawk soaring in the air distinguishes a lark from the 

 similarly-coloured ground ten times farther than the eye of man 

 or dog can reach ; and from a height at which he himself is 

 totally lost to human vision, the falcon pounces down upon the 

 lizard or the field-mouse which he has chosen for his repast. 



Sweeping in majestic circles through the skies, the lammer- 

 geier embraces at a glance a whole world of Alpine solitudes 

 and glaciers. The quadrupeds which inhabit these high regions 

 the goat or the chamois heedlessly graze the mountain herbage, 

 unconscious of the enemy above; but suddenly, with folded 



