BIRDS 



109 



possible time, which excites our admiration to-day, was 

 gradually developed. The faster the birds travelled, 

 the more time they had for breeding, the more young 

 they could bring into the world, and the more quietly 

 they could rear them. Hence the speed of the flight 

 was constantly increased by natural selection. 



Gatke raises the difficulty that every blue- throat that 

 must perform this rapid flight lives on the ground, and 

 never really makes use of its wings except during 

 the migration. Just as a man's arm becomes weaker if 

 it is not used, so it must be with the blue-throat, and we 

 cannot see how it will be able to make the enormous 

 distance. 



It is true that disuse enfeebles an organ, but as only 

 those blue-throats survived that flew quickly in the 

 migration, their strength was increased so much by 

 natural selection that disuse could not lessen it much 

 at least, not enough to incapacitate them from flight. One 

 thing is clear. If the impairing of the flying-power by 

 disuse was inherited, the blue-throats would certainly 

 become weaker and weaker. But this is not the case, 

 and so it is clear that Gatke does not impugn natural 

 selection, as he supposes, but the Lamarckian 

 principle, by his difficulty. We will bear in mind 

 this first case in which the principle conflicts with 

 reality. 



The second effect of natural selection was to increase 

 more and more the bird's power of presentiment. The 

 wanderers must not wait for the snow and ice to tell 

 them that the winter has come, but must take their flight 

 before these appear. Thus the marmot foresees the 



