EXTENSlOiVS OF DxVK\VI.\i;SM 205 



utility to tlie individual; between which and the jooint of ef- 

 fective hurtfulness there may be a considerable margin. In 

 this way w^e have a quite intelligible exi)lanation of the enor- 

 mous development of feathers or decorative pi Limes in so many 

 birds, enormous horns in deer and antelopes, huge tusks in 

 elephants, and huge canine teeth in other quadrupeds. Tliis 

 view is supported by the suggestive fact, that many of these 

 appendages are retained only for a short period, during the 

 breeding season, when vigour is greatest and food most abun- 

 dant, and when therefore they are least injurious. 



Again, when acting in an opposite direction, the theory 

 serves to explain the rapid dwindling and final disappearance 

 of some useless organs, which mere disuse is hardly sufficient 

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

 mentary wings of the Apteryx, the toothless beak of birds, ete. 

 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 deter- 

 minants to be crowded out by the competition of those of ad- 

 jacent parts, the increased development of which was ad- 

 vantageous. 



By this very ingenious, but, though speculative, highly 

 probable hypothesis, extending the s]:)hero of c<inipetition for 

 nourishment and survival of the fittest from \\\o nTgani>=m as 

 a whole to some of its elementarv vital units, Professor Weis- 

 mann has, I think, overcome the one real diHieulty in the in- 

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

 marvellous details, in tenns of normal .variation and survival 

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

 increase beyond the useful limit which Dr. Woodward has rt^ 

 ferred 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 chap- 

 ter. 



