NATURAL SELECTION SINCE DARWIN 83 



case very frequently cited. Herbert Spencer men- 

 tions it in his controversy with Weismann over the/ 

 comparative importance of natural selection and in- 

 heritance of acquired characters as factors in evolu- 

 tion. In the various species of whales in existence at 

 present the hind legs are missing; vestiges of them 

 can, however, he found in the shape of certain bones, 

 (the pelvic bone in some varieties, the femoral in 

 others), hidden under the skin. Their weight is not 

 over 1/900000 of the total weight of the animal. As 

 whales are descended from terrestrial mammals, their 

 limbs must have become progressively atrophied. 

 Was their atrophy due to natural selection? Or is it 

 to the advantage of the animal that the useless bones 

 should decrease in size? Possibly, if we consider that 

 a saving in nutrition would thus be effected. 



When the terrestrial ancestor of the whale was 

 slowly evolving toward its present state, there must 

 have been, Spencer thinks, an enormous increase in 

 the size of the body, caused by habitual overfeeding. 

 | In the embryo, as in the growing animal, there must 

 have been chronic plethora. Why, then, should not 

 the disused parts have profited by this oversupply of 

 nutritive materials? Even if we admit that a saving 

 in food became imperative at a certain time, this ad- 

 vantage could not have been noticeable except in the 

 primary stages of this gradual decrease. In the pres- 

 ent age a whale's femoral bone weighs one ounce; 



