Origin of Species Variorum

Comparison with 1872

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 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 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 Disuse of Parts, as controlled by Natural Selection .

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

well be conceived; and, on the other hand, of different varieties being produced from the same species under apparently the same conditions. Such facts show how indirectly the conditions of life .. act. 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 as these incline me to lay very little weight on the direct action of the conditions of life. Indirectly, as already remarked, they seem to play an important part in affecting the reproductive system, and in thus inducing variability; and natural selection will then accumulate all profitable variations, however slight, until they become plainly developed and appreciable by us.

In a far-fetched sense, however, the conditions of life may be said, not only to cause variability, but likewise to include natural selection; for it depends on the nature of the conditions whether this or that variety shall be preserved. But we see in selection by man, that these two elements of change are essenitally distinct; the conditions under domestication causing the variability, and the will of man, acting either consciously or unconsciously, accumulating the variations in cretain definite directions.

Effects of Use and Disuse, 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 strengthens and enlarges certain parts, and disuse diminishes them; and that such modifications are inherited. Under free nature, we can 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 have structures which can be ex- plained