362 



CHAPTEK XXIV. 



Amphibia.* 



The globe tliat we inhabit is usually said to be 

 made up of land and water, and, perhaps, for the 

 purposes of the geographer, such a division is all 

 that is requisite. A little reflection, however, will 

 convince the naturalist that a very considerable 

 portion of the world around us can scarcely be re- 

 ferred to either of these geographical sections. That 

 there are extensive marshes, for example, equally 

 unfit to be the habitation of aquatic animals, as of 

 creatures adapted to a purely terrestrial existence ; 

 that some localities may be alternately deluged with 

 water and parched with drought, thus the margins of 

 our lakes, the banks of our rivers, and the shallow 

 pools and streamlets of warm climates can only be 

 adequately populated by beings of an amphibious 

 character, alike capable of living in an aquatic or in 

 an aeriform medium, and combining in their structure 

 the conditions necessary for enabling them to reside 

 in either element. 



Aquatic animals, strictly so called, breathe by 

 means of gills ; to adapt a vertebrate animal to 

 respire air, it must be provided with lungs, consisting 

 of membranous bags more or less divided internally 

 into numerous cells, over which the blood-vessels 

 spread like an admirable net-work, fitted for ap- 

 propriating oxygen from the air of the atmosphere 

 instead of from water. But if a creature is destined 

 to live both in air and in water, it must obviously 

 be provided with both gills and lungs coexistent, 

 either of which may be employed in conformity with 

 the necessities of the moment. We cannot, therefore, 

 be surprised that, in the lowest Eeptiles, this is 

 literally the arrangement adopted ; that they respire, 



'■^ afjLcpis, amphis both : /3iow, bioo, to live — living in two elements. 



