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(156

Sect V. Laws of variation

to watch & keep constant their deviations in structure. They are freely left to the play of the various laws of growth, the long-continued effects of disuse, & to the tendency to reversion or inheritance.

A part normally developed in any species in an extraordinary degree or manner, in comparison with the same part in allied species, tends to be highly variable.— Several years ago I was much struck with a remark, nearly to the above effect, published by Mr. Waterhouse. I infer, also, from an observation made by Prof. Owen with respect to the length of the arms of the ourang-outang, that he has independently come to a largely nearly similar conclusion. It is hopeless to attempt to bring convince anyone of the truth of this proposition without giving the long array of facts, which I have collected, & which cannot possibly be here introduced. I can only state my conviction that it is a rule of high generality. I am aware of several causes of error, but I hope that I have made due allowance for them. It should be understood that the rule by no means applies to any part however abnormally developed if or when compared in comparison with the same part in its whole class; but

[in the hand of George Howard Darwin:]

Part of the MSS of the Origin of Species by Chas. Darwin

                              G. H. Darwin Oct. 68

[Origin of species, pp. 149-50 with text identical to the draft in bold:
having no power to check deviations in their structure. Thus [p. 150] rudimentary parts are left to the free play of the various laws of growth, to the effects of long-continued disuse, and to the tendency to reversion.
A part in any species in an extraordinary degree or manner, in comparison with the same part in allied species, tends to be highly variable.—Several years ago I was much struck with a remark, nearly to the above effect, published by Mr. Waterhouse. I infer also from an observation made by Professor Owen, with respect to the length of the arms of the ourang-outang, that he has come to a nearly similar conclusion. It is hopeless to attempt to convince any one of the truth of this proposition without giving the long array of facts which I have collected, and which cannot possibly be here introduced. I can only state my conviction that it is a rule of high generality. I am aware of several causes of error, but I hope that I have made due allowance for them. It should be understood that the rule by no means applies to any part, however unusually developed, unless it be unusually developed in comparison with the same part in closely allied species.

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