CHAPTER V 



EVIDENCES FROM MORPHOLOGY 

 (COMPARATIVE ANATOMY) 1 



GEORGE JOHN ROMANES 



The theory of evolution supposes that hereditary characters 

 admit of being slowly modified wherever their modification will render 

 an organism better suited to a change in its conditions of life. Let 

 us, then, observe the evidence which we have of such adaptive modifi- 

 cations of structure, in cases where the need of such modification is 

 apparent. We may begin by again taking the case of the whales and 

 porpoises. The theory of evolution infers, from the whole structure 

 of these animals, that their progenitors must have been terrestrial 

 quadrupeds of some kind, which gradually became more and more 

 aquatic in their habits. Now the change in the conditions of their 

 life thus brought about would have rendered desirable great modifica- 

 tions of structure. These changes would have begun by affecting the 

 least typical — that is, the least strongly inherited — structures, such 

 as the skin, claws, and teeth. But, as time went on, the adaptations 

 would have extended to more typical structures, until the shape of 

 the body would have become affected by the bones and muscles 

 required for terrestrial locomotion becoming better adapted for 

 aquatic locomotion, and 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. i). 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 completely fish-like in shape 

 (Fig. 12). But profound as are these alterations, they affect only 



1 From G. J. Romanes, Darwin and after Darwin (copyright 1892). Used by 

 special permission of the publishers, The Open Court Publishing Company. 



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