256 OUTLINES OF EVOLUTIONARY BIOLOGY 



the swim-bladder have become highly vascular and it serves as 

 an organ of respiration. Air is taken into it through the mouth, 

 so that the blood circulating in its wall is oxygenated, and the 

 arrangement of the heart and blood-vessels has become modified 

 accordingly. Functional gills, however, are still present. 



In the amphibians the swim-bladder has become completely 

 converted into a pair of lungs, and in this way was rendered 

 possible that great step in the evolution of the vertebrates, the 

 migration from water to land. Thus in the air-breathing verte- 

 brates the gills have been entirely supplanted and replaced 

 functionally by the lungs, which are still to be regarded as 

 homologous with, or morphologically equivalent to, the air- 

 bladder of fishes. 



The hand of man affords another beautiful example of change 



FIG. 110.- The Australian Mild-Fish, Neoceratodus forsleri, greatly reduced. 

 (From British Museum Guide.) 



of function from locomotion to prehension but it has under- 

 gone surprisingly little structural modification in the process. 

 The proboscis of the elephant is also a very efficient organ o 

 prehension, but it is formed by enormous elongation of the nose, 

 which is primarily an organ for conveying air to and from the 

 olfactory organs and lungs. Still more remarkable is the con- 

 version of muscle fibres in the torpedo and the electric eel 

 (Gymnotus) into powerful electric batteries, capable of giving 

 severe shocks and therefore valuable as weapons of offence and 

 defence. In this case the change of function is accompanied by 

 very profound modifications in structure. 



Wherever we turn we find that novel requirements are met, not 

 by the sudden creation of new organs, but by the gradual modifi- 

 cation of old ones. As we have already said, the organism has 

 to do its best with the material which it has inherited from its 

 ancestors ; and yet the power of living protoplasm to meet every 

 new requirement by a suitable modification of bodily structure 



