EVOLUTION AND EXTERNAL APPEARANCE 67 



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have migrated to the top of the head. In very young embryos (4-5 mm. 

 long), the nostrils are still in the normal mammalian place, but by the 

 time the foetus is 22 mm long, they have taken up their final position. 



Why the nostrils are found where they are is not yet entirely clear, but 

 the explanation must probably be sought in the distribution of body 

 weight, and the consequent position of the animal at the water surface. 

 The specific gravity of the normal mammal, once its lungs are filled with 

 air, is such that its body floats in the water. In man and probably in the 

 anthropoid apes also, the nostrils are submerged when the body is floating, 

 but all other mammals naturally assume a position in which the nostrils 

 lie above the surface, and the animal has no difficulty in breathing. This 

 position is the result of two forces: the force of gravity which pulls the body 

 down, and the upthrust of the displaced water, which pushes the body up. 

 The centre of gravity of ordinary terrestrial mammals always lies behind 

 the point of application of the upward thrust (centre of buoyancy) , and 

 the two forces therefore form a couple, with the result that the animal is 

 tilted upwards until its centre of gravity comes to lie perpendicularly 

 below the point of application of the upward force. When that happens, 

 the animal will float obliquely, a position that greatly helps to raise the 

 nostrils out of the water. 



Since whales and dolphins have such an unusual shape, their case 

 differs from that of all other mammals. First, they have no hind legs, and 

 while the tail is thin and light, the head is exceptionally large and heavv. 

 Moreover, the lungs (which are very light, particularly when they are 

 filled with air) stretch far back into the thorax (Fig. 79) . For these reasons, 

 the centre of gravity may be assumed to lie more at the front than it does 



Figure sg. Skeleton of a dolphin-like Archaeocete, Dorudon osiris (Dames), reconstructed 



from bones in museums in Stuttgart and Munich, and found in 35-milUon-year-old deposits in 



Fayum {Egypt). {Slijper, 1936.) 



