138 EFFECTS OF USE AND DISUSE. 



other hand, of dissimilar varieties being produced under 

 apparently the same external conditions. Again, innumer- 

 able instances are known to every naturalist, of species 

 keeping true, or not varying at all, although living under 

 the most opposite climates. Sach considerations as 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 like- 

 wise 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 accumu- 

 lates the variations in certain direction; and it is this latter 

 agency which answers to the survival of the fittest under 

 nature. 



BEFBCTS OF THE INCREASED USE AST) DISUSE OF PARTS, AS 

 CONTROLLED BY NATURAL SELECTION. 



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

 there can be no doubt that use in our domestic animals has 

 strengthened and enlarged certain parts, and disuse dimin- 

 ished 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. Cunningliam, 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 



