274 THE WORLD OF LIFE chap. 



to explain ; such are the lost hind limbs of whales, the , 

 rudimentary wings of the Apteryx, the toothless beak of I 

 birds, etc. In such cases, after natural selection had reduced 

 the part to a rudimental condition, any regrowth would be 

 injurious, and thus determinants of increased vigour would 

 be suppressed by the non-survival of the adult, leaving the 

 weaker determinants to be crowded out by the competition i 

 of those of adjacent parts, the increased development of i 

 which was advantageous. 



By this very ingenious, but, though speculative, highly 

 probable hypothesis, extending the sphere of competition for 

 nourishment and survival of the fittest from the organism as; 

 a whole to some of its elementary vital units, Professor 

 Weismann has, I think, overcome the one real difficulty in the 

 interpretation of the external forms of living things, in all their 

 marvellous details, in terms of normal variation and survival 

 of the fittest. We have here that " mysterious impetus " to 

 increase beyond the useful limit which Dr. Woodward has 

 referred to in his address already quoted, and which is alsc 

 a cause of the extinction of species to which Mr. Lydekkei 

 referred us, as quoted towards the end of the preceding chapter 



Illustrative Cases of Extreme Development 



Two examples of this extreme development have not 

 I think, yet been noticed in this connection. The wonderfu 

 long and perfectly straight spirally twisted tusk of tfr 

 strange Cetaceous mammal, the narwhal, is formed by ai 

 extreme development, in the male only, of one of a pair c 

 teeth in the upper jaw. All other teeth are rudimentary, a, . c 

 is the right tooth of the pair of which the left forms tb 

 tusk, often 7 or 8 feet long, and formed of a very fidwft: 

 heavy ivory. The use of this is completely unknown, fc 

 though two males have been seen playing together, apparently 

 with their tusks, they do not fight, and their food, being sma 

 Crustacea and other marine animals, can have no relatic 

 to this weapon. We may, however, suppose that the tu< 

 was originally developed as a defence against some enem 

 when the narwhal itself was smaller, and had a wider ran£ 

 beyond the Arctic seas which it now inhabits ; and wh< 

 the enemy had become extinct this strange weapon went « 





