THE ARGUMENTS FROM DISTRIBUTION. 395 



fenown to survlye for days out of tlie water. But the facts of 

 greatest significance are furnislied by an allied class of 

 Fertehrata, almost peculiar to habitats of this kind. The 

 Amphibia are not, like fish, habitually found in waters that 

 are never partially or wholly dried up ; but they nearly all 

 inhabit waters which, at certain seasons, evaporate, in great 

 measure or completely — ^waters in which most kinds of fish 

 cannot exist. And what are the leadin.g structural traits of 

 these Amphibia? They have two respiratory systems — 

 pulmonic and branchial — ^variously developed in difterent 

 orders ; and they have two or four limbs, also variously de- 

 veloped. Further the class Amphibia consists of two groups, 

 in one of which this duality of the respiratory system is 

 permanent, and the development of the limbs always incom- 

 plete ; and in the other of which the branchiae disappear as 

 the lungs and limbs become fully developed. The lowest 

 group, the ,Perennib7^anchiatay have organs homologous T\dth 

 the air-bladders of fishes, transformed in various degrees 

 into lungs, until ^' in the Sireii, the pulmonic respiration is 

 more extensive and important than the branchial ; '^ and to 

 these creatures, having a habitat partially aerial and partially 

 aquatic, there are at the same time supplied, in the shallow 

 water covering soft mud, the mechanical conditions which 

 render swimming difficult and rudimentary limbs useful. 

 In the higher group, the Gaducibranchiata, we find still more 

 suggestive transformations. Having at first a structure re- 

 sembling that which is permanent in the perennibranchiate 

 amj)hibian, the larva of the caducibranchiate amphibian, 

 pursues for a time a similar life; but eventually, the 

 changes are carried further in the same direction : the respir- 

 ation of air, originally supplementary to the respiration of 

 water, predominates over it more and more, till it replaces it 

 entirely ; and an additional pair of legs is produced. This 

 having been done, the creature either becomes, like the Triton^ 

 one which quits the water only occasionally ; or, like the 

 Frog, one which pursues a life mainly terrestrial, and returns 



