46 COMPARATIVE ANATOMY OF 



is not long gone by when it was supposed that these 

 Amphibia differed from all other animals, in that it 

 was to them a matter of indifference whether they 

 lived on land, and breathed the common air of 

 heaven, or resorted to the water, and there carried 

 on the process corresponding to that of respiration, 

 as do the fishes in the sea. It is generally known 

 that both in quadrupeds and fishes there is a kind 

 of double circulation of the blood, the greater, mov- 

 ing round the whole body, for the purpose of its 

 growth and regular nourishment, and the lesser, con- 

 fined to the lungs or the gills, having for its object 

 the purification of the blood, that it may be fitted 

 for its proper uses. It was very evident that, when 

 living in the water, these Amphibia could no more 

 use their lungs than man could do, and it was 

 equally well known that they had no gills, by which 

 they could live as fish do ; and the question there- 

 fore occurred, By what peculiar arrangement is it 

 that these animals live in water without gills and 

 without air ? Nor was it long before an explana- 

 tion of the phenomenon was offered. The two cir- 

 culations above alluded to do not communicate with 

 each other, generally, throughout their circuit ; but 



that previous to Linnaeus' death, a single animal was discovered 

 which could live not only on land but also in water. This was 

 the Lizard Siren of Carolina, which is possessed of true lungs and 

 true branchiae, and can use them severally on land and in water. 

 Since that date a few other animals have been discovered similarly 

 constituted ; and this has led Naturalists to arrange them in a 

 separate class or subdivision. 



