18 JANUARY 1873, Page 12




SIR,—Any one interested in the subject to which you allude at p. 42 of your last number, namely, the relative importance in causing modifications of the body or mind, on the one hand of habit or of the direct action of external conditions?. and on the other hand of natural or artificial selection, will find this subject briefly discussed in the second volume (pp. 301-315) of my "Variation of Animals and Plants under Domestication." I have there given a considerable body of facts, chiefly in relation to acclimatisation, which presents the greatest difficulty in the pre- sent question ; and it may be inferred from these facts, firstly, that variations of a directly opposite nature, which would be liable either to preservation or elimination through natural selec- tion, not rarely arise in organisms long exposed to similar condi- tions; and secondly, that habit, independently of selection, has often produced a marked effect. But it is 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 tend to concur, for the individuals which inherit in the strongest manner any useful habit will commonly be preserved.

Take, as an instance, the fur of quadrupeds, which 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 it is extremely difficult 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 and collected many facts, showing the potent influence of habit and of the use or disuse of parts on organic beings ; but there are numberless peculiarities of structure and of instinct (as in the case of sterile neuter insects) which can- not be thus accounted for. He would be a bold man who would attempt to explain by these means the origin of the exsertile claws .and great canine teeth of the tiger ; or of the horny lamellse on the beak of the duck, which are no well adapted for sifting water. Nor would anyone, I presume, even attempt to explain the development, for instance, of the beautifully plumed seeds of the dandelion, or of the endless contrivances which are necessary for the fertilisation of very many flowers by insects, through gradually acquired and inherited habit, or through the direct action of the external conditions of life.—I am, Six, &c., Down, Beckenham, Kent, Jan. 11, 1873. CHARLES DARWIN.