200 THE AIR-BLADDER OF FISHES 



or the development of the hind Hmbs in whales. We should not 

 discover the original history of these organs, but only that of 

 their extreme modification, or gradual suppression. 



Mr. Morris next asks us to imagine what were the conditions 

 of life under which this organ was developed, and what were the 

 later conditions which rendered it in great measure or entirely 

 useless. The question takes us back to Devonian and Silurian 

 geological periods. In this era the seas were thronged with fishes 

 of two distinct orders — the Elasmobranchs and the Ganoids, the 

 former without, the latter with, an air-bladder. This difference in 

 organisation was probably the result of some marked difference in 

 their life-habits. The Ganoids may have inhabited poorly aerated 

 waters, or waters otherwise ill-adapted to breathing, while the 

 Elasmobranchs may have had their primordial habitat in clearer 

 and purer waters. 



But there were other conditions which may have been the main 

 influencing causes of the development of an air-breathing organ. We 

 know that the land was habitable for long ages ere it gained any 

 vertebrate inhabitants. The presence of insects in Devonian and 

 Silurian strata proves this. Long ages passed during which we have 

 no evidence of land animals higher than insects or snails, and the 

 earth must have possessed much food material, both vegetable and 

 animal, available for a larger population than it possessed. It is 

 hardly probable that the active fish of the early seas made no 

 effort to obtain a share of this food. As many fishes now leave 

 the water temporarily for the land, in search of food, notwithstand- 

 ing the many dangerous enemies which lie in wait for them, it is 

 only reasonable to suppose that many fishes may have done so in 

 remote ages, when no such redoubtable foes awaited them. Such 

 fishes as left the sea for the land would find their enterprise richly 

 repaid by ample food, and would therefore have had a powerful 

 inducement to continue the habit. Other influences may be 

 inferred to have been at work then, as they are now. Foul and 

 muddy water, and the drying-up of pools, which now cause fish to 

 breath atmospheric air, must have been causes as likely to be 

 active in Devonian times as now. 



At the opening of the carboniferous era there may have been 

 many lung-and-gill breathing Dipnoi, finned Batrachians as we 



