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To the Editor

Sir
Any one interested in the subject, to which you allude at p. 42 of your last number, namely, the relative importance in the causing modifications of the body & or mind, on the one hand of habit or uses or of the direct action of external conditions, & on the other hand of natural or artificial selection, will find this subject briefly discussed in the second volume (p. 301 — 315) of my Variation of Animals & Plants under Domestication. I have there given a considerable body of facts, chiefly in relation to acclimatisation, which presents the much the greatest difficulty in the present question; & it may be inferred from these facts, firstly, that variations of a directly opposite nature, which wd be liable either to preservation or elimination through natural selection, not rarely arise in organisms long exposed to similar conditions; & secondly that habit, independently of selection, has (often produced a marked effect. But it is

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[Delahousse]

most difficult, as I have insisted in many of my works, though in some cases possible, to discriminate between the results of the two processes,. Both naturally tend to go on together concur, for the individuals which inherit (any useful habit) in the strongest manner will commonly be preserved.

Take as an instance the fur of quadrupeds, which always grows thickest in the individuals living far north: now there is reason to believe that weather acts directly on the skin with its appendages, but now it is extremely difficult if not to judge how much of the effect ought to be attributed to the direct action of a low temperature, and how much to the best protected individuals of many generations having survived during the severest winters. I have made many observations & collected many facts, showing

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the potent influence of habit & of the use or disuse of parts on organic beings; but there are numberless peculiarities of structure & of instinct (as in the case of sterile neuter insects) which cannot be thus accounted for. He would be a bold man who would thus attempt to explain by these means the origin of the exsertile claws & great canine teeth of the tiger; or of the horny lamellæ on the beak of the common duck, which are so well adapted for sifting water. Nor would anyone, I presume, even attempt to explain through the force of inherited habit or through the direct action of the surrounding conditions the formation development, for instance, of the beautifully plumed seeds of the dandelion, or of the endless contrivances in every flower which are necessary for their fertilisation by means of insects. by insects of very many many flowers, through gradually acquired & inherited habit, or through the direct action of the external conditions of life.

Charles Darwin

Down Beckenham Kent

Jan 11'

Only the signature is in Darwin's handwriting, the remainder is in the hand of Emma Darwin.