100 EFFECTS OF USE AND DISUSE. [CHAP. V. 



species have thicker and better fur the f urther north they live ; 

 but who can tell how much of this difference may be due to the 

 warmest-clad individuals having been favoured and preserved 

 during many generations, and how much to the action of the 

 severe climate ? for it would appear that climate has some direct 

 action on the hair of our domestic quadrupeds. 



Instances could be given of similar varieties being produced 

 from the same species under external conditions of life as different 

 as can well be conceived ; and, on the other hand, of dissimilar 

 varieties being produced under apparently the same external 

 conditions. Again, innumerable instances are known to every 

 naturalist, of species keeping true, or not varying at all, although 

 living under the most opposite climates. Such considerations nc 

 these incline me to lay less weight on the direct action of the 

 surrounding conditions, than on a tendency to vary, due to causes 

 of which we are quite ignorant. 



In one sense the conditions of life may be said, not only to 

 cause variability, either directly or indirectly, but likewise to 

 include natural selection, for the conditions determine whether 

 this or that variety shall survive. But when man is the selecting 

 agent, we clearly see that the two elements of change are distinct ; 

 variability is in some manner excited, but it is the will of man 

 which accumulates the variations in certain directions ; and it is 

 this latter agency which answers to the survival of the fittest 

 under nature. 



Effects of the increased Use and Disu&e of Parts, as 

 controlled by Natural Selection. 



i From the facts alluded to in the first chapter, I think there can 

 I be no doubt that use in our domestic animals has strengthened 

 ' and enlarged certain parts, and disuse diminished them ; and that 

 "such modifications are inherited. Under free nature, we have no 

 standard of comparison, by which to judge of the effects of long- 

 continued use or disuse, for we know not the parent-forms ; but 

 many animals possess structures which can be best explained by 

 the effects of disuse. As Professor Owen has remarked, there is 

 no greater anomaly in nature than a bird that cannot fly; yet 

 there are several in this state. The logger-headed duck of South 

 America can only flap along the surface of the water, and has its 

 wings in nearly the same condition as the domestic Aylesbury 

 duck : it is a remarkable fact that the young birds, according to 

 Mr. Cunningham, can fly, while the adults have lost this power. 

 As the larger ground-feeding birds seldom take flight except to 

 escape danger, it is probable that the nearly wingless condition of 

 several birds, now inhabiting or which lately inhabited several 

 oceanic islands, tenanted by no beasts of prey, has been caused by 



