Structural Adaptations 



57i 



cover its back, and in this position only the experienced eye 

 can detect its whereabouts. The similarity of habit in this 

 respect to those of the crocodile and frog, will be at once 

 obvious. 



We have here a very definite example of an acquired 

 adaptation transmitted by inheritance. The orbits of the 

 land-living relations of the Hippopotamus are not so placed, 

 and we may safely believe that it has been as the result of 

 constant efforts to lift the eyes that they have been by slow 

 degrees made to travel upwards into their present position. 



There is, however, another and yet more remarkable 

 adaptive peculiarity in the Hippo orbit to which we must 

 ask attention. * Unfortunately it is not possible to display 

 it in a photograph, and the skull itself must be consulted in 

 order to realise its nature. There exists inside the orbit, 

 so placed as to lift and support the eye, a large, hollow, bony 

 cushion. It is formed by a layer of bone, hardly thicker than 

 paper, and so delicate that in most museum-specimens it has 

 been broken away. In some instances of adult or old animals 

 it is as large as a duck's egg, whilst in young ones it is but 

 small. To this structure, well known to zoologists, the name 

 of orbital bulla has been given. The bone from which it is 

 developed is, of course, present in all mammals. It varies 

 very much in size, and in many species forms a considerable 

 part of the floor of the orbit, but lies flat and shows no 

 tendency to form a bulla. . There are traces of bulla forma- 

 tion in the orbit of the Giraffe, and it may be remembered 

 that that animal in browsing on the twigs of trees has 

 occasion for the elevation of its eyeballs. 



Perhaps no more definite and instructive example of adapta- 

 tion of structure to the needs of the animal can be pointed out 

 than the elevation of the orbit in the skull of the Hippo- 

 potamus and the development of the bulla in its floor. It 

 was partly in reference to them that we have insisted on 

 the great value of this skull in all educational museums. 



