10 MANUAL OF NATURAL HISTORY. 



nocturnal, require appropriate organs of vision ; thus 

 we find the pupils of the eyes Very large and sus- 

 ceptible, and that they may not be inconvenienced by 

 the glare of day, the irides remain contracted during 

 that time, leaving but a small part exposed. In the 

 Seals, and more particularly in the Whales, we see 

 the form modified, so as to be entirely adapted for an 

 aquatic life. With the former it becomes necessary 

 that they should at times seek the land or the ledges 

 of rock for repose; their shape therefore has still 

 some resemblance to the terrestrial members of the 

 class to which they belong; while in the latter, des- 

 tined to reside wholly in the water, the external 

 form is fish-like, only, however, purely adaptive, as 

 they still possess the essential character of mammals, 

 having the organs of respiration and circulation so 

 arranged as to enable them to dive down and re- 

 main for a limited period in the depths of the ocean. 

 The Camel, intended to live on barren sandy tracts 

 of dry and heated desert land, where other animals, 

 without some special provision, would perish from 

 thirst, are provided with the means of allaying 

 theirs, by a peculiar modification of the stomach, 

 which thus becomes a reservoir of water sufficient 

 for many days' consumption ; their feet, too, broad, 

 padded, and elastic, are well suited for travelling 

 over such light and yielding soil. The teeth of the 

 Rodents furnish us with another instance. The 

 front, or cutting teeth, by continually growing from 

 a persistent pulp, compensate for the wear to which 

 they are constantly subjected, and the inner part, 



