Page - 147 - in The Origin of Species

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,