Morphology. 5 1 



the whole outline of the animal more fish-like in 

 shape. This is the stage which we actually observe 

 in the seals, where the hind legs, although retaining 

 all their typical bones, have become shortened up 

 almost to rudiments, and directed backwards, so as 

 to be of no use for walking, while serving to complete 

 the fish-like taper of the body. (Fig. 2.) But in the 

 whales the modification has gone further than this 

 so that the hind legs have ceased to be apparent 

 externally, and are only represented internally and 

 even this only in some species by remnants so 

 rudimentary that it is difficult to make out with 

 certainty the homologies of the bones ; moreover, 

 the head and the whole body have become com- 

 pletely fish-like in shape. (Fig. 3.) But profound 

 as are these alterations, they affect only those parts 

 of the organism which it was for the benefit of the 

 organism to have altered, so that it might be adapted 

 to an aquatic mode of existence. Thus the arm, 

 which is used as a fin, still retains the bones of the 

 shoulder, fore-arm, wrist, and fingers, although they 

 are all enclosed in a fin-shaped sack, so as to render 

 them useless for any purpose other than swimming 

 (Fig. 4.) Similarly, the head, although it so closely 

 resembles the head of a fish in shape, still retains 

 the bones of the mammalian skull in their proper 

 anatomical relations to one another; 'but modified 

 in form so as to offer the least possible resistance 

 to the water. In short, it may be said that all the 

 modifications have been effected with the least pos- 

 sible divergence from the typical mammalian type, 

 which is compatible with securing so perfect an 

 adaptation to a purely aquatic mode of life. 



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