A species of plant that has been transferred from one soil or climate to another, frequently undergoes what botanists call "change of habit"--a change which, without affecting its specific characters, is yet conspicuous. In its new locality the species is distinguished by leaves that are much larger or much smaller, or differently shaped, or more fleshy; or instead of being as before comparatively smooth, it becomes hairy; or its stem becomes woody instead of being herbaceous; or its branches, no longer growing upwards, assume a drooping character. Now these "changes of habit" are clearly determined by functional changes. Occurring, as they do, in many individuals which have undergone the same transportation, they cannot be classed as "spontaneous variations." They are modifications of structure consequent on modifications of function that have been produced by modifications in the actions of external forces. And as these modifications re-appear in succeeding generations, we have, in them, examples of functionally-established variations that are hereditarily transmitted. Evidence of analogous changes in animals is difficult to disentangle. Only among domesticated kinds have we any opportunity of tracing the results of altered habits; and here, in nearly all cases, artificial selection has obscured them. Still, there are some facts which seem to the point. Mr. Darwin, while ascribing almost wholly to "natural selection" the production of those modifications which eventuate in differences of species, nevertheless admits the effects of use and disuse. He says--"I find in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in proportion to the whole skeleton, than do the same bones in the wild duck; and I presume that this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parent. The great and inherited development of the udders in cows and goats in countries where they are habitually milked, in comparison with the state of these organs in other countries, is another instance of the effect of use. Not a single domestic animal can be named which has not in some country drooping ears; and the view suggested by some authors, that the drooping is due to the disuse of the muscles of the ear, from the animals not being much alarmed by danger, seems probable." Again--"The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite covered up by skin and fur. This state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection." ... "It is well known that several animals belonging to the most different classes, which inhabit the caves of Styria and of Kentucky, are blind. In some of the crabs the footstalk of the eye remains, though the eye is gone; the stand for the telescope is there, though the telescope with its glasses has been lost.