66 



THE BRITISH NATURALIST. 



[March 



as widely differentiated as existing forms, flourished upon the earth, and 

 were themselves the descendants of a line of ancestors, all vertebrate, who 

 had been gradually varying and developing during the almost immeasur- 

 ably vast epochs during which the old red sandstone beds were being 

 deposited ; not to speak of the lapse of time between that and the Silurian, 

 a period which would be more interesting than any subsequent one, had 

 we but a complete record of its palaeontology, for then was bridged the 

 gap which separated the vertebrates from all other animals, and a stride 

 made greater than any in the biological history of our planet between 

 the formation of the first protist and the birth of the first man. 



The lungs, the organs of respiration of terrestrial vertebrates, occur, as 

 previously mentioned, amongst animals which also possess gills, enabling 

 them to breathe in water and constituting them amphibians in the strict 

 sense of that word. 



It has also been mentioned that one group of fishes, the Dipnoi, 

 possess lungs, and are thus enabled to survive during the long, dry 

 season when the tropical rivers which they inhabit become dry. 



There are only three existing species of these remarkable fishes,whose 

 skeletal structure closely resembles that of the ganoids, a group which 

 certainly does not take high rank, and whose fossil representatives are 

 amongst the very oldest of the vertebrates, some occurring in the 

 Silurian period. The lungs of the higher animals are foreshadowed by 

 the air bladders of fishes, and are but a higher development of them. 

 Like lungs, air bladders arise as diverticula of the walls of the 

 oesophagus, but their functions are widely different, and the arrange- 

 ment of the blood vessels also differs, air-bladders being surrounded by 

 vessels which in approaching contain arterial and in passing away 

 venous blood ; in other words, there is no aeration effected in the air- 

 bladder, speaking generally. The function of the air-bladder is to alter 

 the specific gravity and facilitate the ascent or descent of the fish in the 

 water. It is usual for the air-bladder to arise from the dorsal wall 

 whilst the lungs always arise from the ventral wall of the oesophagus. 

 There are exceptions, however, to this rule, and these exceptions indicate 

 the line of descent of the lung-breathing vertebrates. One of these 

 exceptions is the genus Polypterus belonging to the Crossopterygian 

 section of the Ganoid river fishes ; in it the air-bladder arises from the 

 ventral wall of the oesophagus, exactly as do the lungs of the Dipnoi and 

 all higher animals. We thus see that in some ancient Ganoid, like the 

 existing Polypterus, the air-sac arose on the ventral side of the 

 oesophagus. Under conditions like those of the sub-tropical rivers 

 inhabited by the existing Dipnoi, or Australian, African and South j 

 American mud fishes, this air-sac became physiologically as well as I 

 morphologically a lung, and the passage thence to the batrachian, 

 reptilian, and mammalian lung is made by a number of gradations which 



