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 

 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 

 of those of adjacent parts, the increased development of 

 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 also 

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

 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 wonderful 

 long and perfectly straight spirally twisted tusk of the 

 strange Cetaceous mammal, the narwhal, is formed by an 

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

 teeth in the upper jaw. All other teeth are rudimentary, as 

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

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

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

 though two males have been seen playing together, apparently, 

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

 Crustacea and other marine animals, can have no relation 

 to this weapon. We may, however, suppose that the tusk 

 was originally developed as a defence against some enemy, 

 when the narwhal itself was smaller, and had a wider range 

 beyond the Arctic seas which it now inhabits ; and when 

 the enemy had become extinct this strange weapon went on 



