CiiAi-. V. EFFECTS OF USE AND DISUSE. in<) 



1)ftM» favored and preserved during many generations, and how 

 much lo the action of tlie severe climate ? for it would appear 

 that climate has some direct action on the hair of our domestic 

 (juachupeds. 



Instances could be given of similar varieties being produced 

 from the same species under external conditions of life as dif- 

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

 dissimilar varieties being produced imder apparently the same 

 external conditions. Again, innumerable instances are known 

 to ev(;ry naturalist, of species keeping true, or not varying at 

 all, although living und<'r the most opposite climates. Such 

 considerations as these incline me not to lay much weight on 

 the direct and definite action of the conditions of life; but I 

 fully admit that strong arguments of a general nature may be 

 advanced on the other side. 



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

 cause variability, but likewise to include natural selection ; for 

 the conditions determine whether this or that variety shall sur- 

 yi\i.\ ]3ut when man is the selecting agent, we clearly see 

 that the two elements of change are distinct ; the conditions 

 cause the variability; the will of man, acting either consciously 

 or unconsciously, accumulates the variations in certain direc- 

 tions, and tills answers to the survival of the fittest under 

 nature. 



J^Jfects of Use and Disrise, as controlled hy Natural Selection. 



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

 can be no doubt that use in our domestic animals strengthens 

 and enlarges certain parts, and disuse diminishes them ; and 

 that such modifications are inherited. Under free Nature, we 

 liave no standard of comparison, by which to judge of the 

 elfecls of long-continued use or disuse, for "we know not the 

 parent-fonns; but many animals have structures which- can be 

 (•xi)lain<'d by the efl'ects of disuse. As Prof. Owen has remarked, 

 llierc is no greater anomaly in Nature than a bird that cannot 

 l!y ; yet there are several in this state. The logger-headed duck 

 of South America can only llap along the surface of the water, 

 and has its wings in nearly the same condition as the domestic 

 Aylesbury duck. As the larger ground-feeding birds seldom 

 tak(> fiight except to escap<> danger, I believe that the nearly 

 wingless condition of several birds, which now inhabit or have 

 lately inhabited several oceanic islands, tenanted by no beast 



