EFFECTS OF USE AND DISUSE 147 



duced 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 appar- 

 ently the same external conditions. Again, innumerable in- 

 stances are known to every naturalist, of species keeping 

 true, or not varying at all, although living under the most 

 opposite climates. Such 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 some 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 deter- 

 mine whether this or that variety shall survive. But when 

 man is the selecting agent, we clearly see that the two ele- 

 ments of change are distinct ; variability is in some manner 

 excited, but it is the will of man which accumulates the va- 

 riations 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 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 dis- 

 use, 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 Aylcsbury-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, 



