[Enclosure]³

Letter to G. H. Lewes

Aug. 7 1868.— against organs having been formed by direct action of medium in distinct organisms.—⁴

Chiefly Luminous & Electric Organs & Thorns—

p. 76. I have also in my own mind always taken I trust nearly the same view, as you maintain, about the luminous organs of insects or the electric organs of fishes; but if you maintain that these organs are the direct results of the conditions of life, or the medium independently of natural selection I cannot follow you.⁵

In my opinion there will always be confusion, in every discussion, as long as the action of the external & internal conditions of of life in causing variability or modification are confounded or mixed up with natural or artificial selection.— In the formation for instance of the breed of pouter pigeon; the conditions cause the successive variations, but man makes the breed by selection; and this distinction equally holds under nature, though the conditions [instead] of the will of man here determine the kind of selection. I could almost as soon admit that the whole structure of, for instance, a woodpecker, had thus originated; circumstances which cannot directly affect structure seems to me ought to impossible. Such organs as those above specified seem to me much too complex and generally too well & so complexly coordinated with the whole organisation, for the admission that they result from conditions independently of named selection. The impression which I have taken studying nature is strong, that in all cases if we could collect all the forms which have ever lived, we sh^d have a clear gradation from some most simple beginning. If similar conditions sufficed without the aid of natural selection to give similar parts or organs, independently of blood relationship, I doubt much whether we sh^d have that striking harmony between the affinities, embryological development, geograph. distrib, & geolog. succession of all allied organisms. We sh^d be much more puzzled than we now are how to class in a natural method many forms. It is puzzling enough to distinguish between resemblance due to descent & to adaptation; but (fortunately for naturalists) owing to the strong power of inheritance, & to excessively complex causes and laws of variability, when the same end or object has been gained, somewhat different parts have generally been modified, or modified in a different manner, so that the resemblances due to descent & adaptation can commonly be distinguished.

I sh^d like just to add, that we may understand each other, how I suppose the luminous organs of insects for instance, to have been developed, but I depend on conjectures; for so few luminous insects exist that we have no means of judging, & [through] the preservation to the present day of slightly modified forms, of the probable gradations, through which the organs have passed.⁶ Moreover we do not know of what use these organs are. We see that the tissues of many animals as certain centipedes in England are liable, under unknown conditions of food, temperature, &c, to become occasionally luminous; just like the pyrophorus mixture gives in [churned] Bog   Such luminosity having been advantageous to certain insects, the tissues, I suppose, become specialised for this purpose in an intensified degree, in certain insects in one part in other insects in other parts of the body. Hence I believe that if all extinct insect-forms could be collected, we sh^d have gradations from the Elateridæ⁷ with their highly & constantly luminous thorax’s & from the Lampyridæ⁸ with their highly luminous abdomens to some ancient insects, occasionally luminous like the centipede.

I do not know but suppose that the microscopical structure of the luminous organs in the most different insects is nearly the same, and I should attribute to inheritance from a common progenitor that the similarity of the tissues, which under similar conditions, allowed them to vary in the same manner, & thus through natural selection for the same general purpose, to arrive at the same result.

Mutatis mutandis I sh^d apply the same doctrine to the electric organs of fishes; but here I have to make in my own mind the violent assumption that some ancient fish was slightly electrical without having any special organs for the purpose. It has been stated on evidence not trustworthy, that certain reptiles are electrical. It is, moreover, possible that the so-called electric organs, whilst in a condition not highly developed may have subserved some distinct function: at least, I think Matteucchi c^d detect no pure electricity in certain fishes provided with the proper organs.—⁹

In one of your letters you alluded to nails, claws, hoofs, &c.¹⁰ From their perfect coadaptation with the whole rest of the organisation, I cannot admit that they could have been formed by the direct action of the conditions of life. H. Spencer’s view that they were first developed from inherited skin, the result of pressure on the extremities, seems to me probable.¹¹

In regard to thorns & spines, I suppose that stunted and [small] hardened processes were primarily left by the abortion of various appendages, but I must believe that their extreme sharpness & hardness is the result of fluctuating variability & “the survival of the fittest.” The precise form, curvature & colour of the thorns I freely admit the to be the result of the laws of growth of each particular plant, or of their conditions internal & external. It w^d be an astounding fact if any varying plant suddenly produced, without the aid of reversion or selection perfect thorns. That natural selection would tend to produce the most formidable thorns will be admitted by everyone who has observed the distribution in S. America & Africa (vide Livingstone) of thorn-bearing plants, for they always appear where the bushes grow isolated & are exposed to the attacks of mammals.¹² Even in England it has been noticed that all spine-bearing & sting-bearing plants are palatable to quadrupeds, when the thorns are crushed.— With respect to the Malayan climbing Palm, what I meant, to express is that the admirable hooks were perhaps not first developed for climbing; but having been developed for protection were subsequently used and perhaps further modified for climbing.¹³

As the view which you have taken on the subject here discussed seems firmly fixed, I do not suppose that anything which I have written, even supposing it mainly true, will have much influence; for I know by my own experience that a conclusion slowly arrived at cannot be quickly changed. But I have liked to say my say, & I hope it will not trouble you too much to read and consider it; but this discussion ought to have been been much fuller.