My dear Charles,

Having been somewhat indisposed when you were here yesterday (a thing very rare with me) I probably failed in making my meaning perfectly clear as to some particulars, & omitted others which had occurred to me. I take up a sheet therefore, & will briefly put down a few of these comments or suggestions; for the bare chance of this being useful to you, either in the second or future editions.

A closer definition, seems to me to be required, of the phrase “natural selection”; the governing principle of the work. From different passages in the volume, it might be variously interpreted, as a necessity, as an accident, as an instinct, as intellectual volition. From the definition in p. 81, I infer that necessity is mainly concerned in the principle of change supposed. If so, would it not be well to alter some of the phrases which create ambiguity, such as “picking out with unerring skill” “working for the good of each being” “natural selection having taken advantage of” “for the sake of eliminating &c &c²

If this principle, so limited to a relative struggle for life, be applied to the variations from certain primitive forms, it may explain the progressive development into higher forms or conditions of some series in the scale of being. But does it explain the arrest of those other forms of lower vitality, which crowd, & have ever crowded, earth & sea with their microscopic multitudes?

The intellect & interests of man have been successful in altering the external conditions, & modifying the instincts of many animals; & in rendering such changes hereditary. But has the skeleton, or have the organs of nutrition, of circulation, of respiration, of secretion, of generation, of nervous power in these cases ever undergone any essential change?

This leads me further to remark that the argument rests generally too much on what may be called external conditions. Artificial selection looks to these almost solely. Natural selection, to satisfy the demands of the theory, must go much further, & explain, or give some plausible rationale, of the changes which have occurred in the different scales or groups of animal life, in the organs subserving to the functions denoted above. This has been partially done; but not, I think, adequately to the demand. I will give only an example or two, occurring at the moment, instar multarum. How explain by natural selection, the change from a Heart with two cavities to a Heart with four & with the appropriate valves? Or how, the sudden appearance of the Corpus Callosum or great Commissure of the Brain, when we get above the Marsupials in the scale of the Mammalia?

(I am the rather led to this latter instance, by the interest I attach to the question of the Doubleness of the Brain, & the connexion by commissures of its equal & similar sides—a point curiously neglected by physiologists— You will find a Chapter on the subject in my “Mental Physiology”)³

Connected with the above, I would refer, (as I did slightly yesterday) to that passage in p. 189, where you put your theory, perhaps needlessly, to a severe test⁴ The test, so put, I must answer by saying that I cannot conceive any possible chances,—however multiplied by time & succession of generations, which could have produced the chain of little bones within the tympanum of the Ear, detached from all other bony structure, & so striking in their individual peculiarity, that if ever one might venture to affirm a final cause, while ignorant of the mode of its fulfilment it would be here— Further, is it possible to conceive the Eustachian tube, so obvious in its use, as an accident, perpetuated by natural selection?

Regarding the Eye, I am obliged to avow that I think your reasoning does not reach the case; & the comparison to the gradual progress of the Telescope is not satisfactory, because (notwithstanding one ambiguous sentence) your understanding of natural selection, does virtually exclude reason from any part of the resultThat the various parts of the Eye, including the Humours, could reach their highest state of completeness & mutual adaptation, by any means short of prevision in their formation, I cannot comprehend. No scheme of casualties, granting indefinite time for their multiplication, seems to me capable of reaching this result

As applied to the test of p. 189, I spoke also of the Electrical animals, as a case for which I cannot see any even approximate solution. The poisonous animals offer difficulties of the same kind,—as also those animals, the Spider & Silk Worm for example, which secrete Chemical fluids, of specific composition, under correlated conditions of structure fitted to give these fluids their proper function in the life of these animals. I confess I never look at a Spider’s web, & at the astute artist, armed with its strange instincts, sitting amidst its work, without a certain feeling of awe at the many mysteries it involves.

I think I stated my impression that you had not been explicit enough on the subject of Instincts, as connected with your theory. If all animal forms be derived from one, or very few primitive forms, Instincts must have followed, & been caused by, the successive steps of structural change That habits acquired often become hereditary, & no longer to be distinguished from Instincts, I fully believe.⁵ But still I think there are numerous cases in the history of Instincts, where the peculiarity of the individual instinct is such, that we are almost compelled to regard the bodily structure as the instrument & not the cause, of the Instinct. The whole matter is full of difficulty; from which Sir I Newton seeks shelter in his 31^st Query, by supposing the Deity “the present moving principle” in all Instincts.—⁶ Notwithstanding that Intelligence & Instincts seem to stand in opposition, or rather in inverse ratio to each other throughout animal life, I have a strong impression that the tendency of future research will be to bring them into closer convergence; & this, even though we add little to our knowledge of the essence of either.

I mentioned the specific differences of the blood corpuscles in form & size, as presenting another difficulty. But I admit that we know too little yet of this curious part of physiology, to justify any argument from it. I do not recollect that you anywhere advert especially to the blood, & to the distinctions of warm & cold, or red & colourless blooded animals; & yet seeing the importance of the subject I think you must have done so.⁷ The questions as to the blood are the most interesting in all physiology, save only those which relate to the nervous system; & I cannot marvel much at John Hunter’s regarding this wonderful fluid as an especial seat of vitality

I would here remark further, that I think you have spoken too little of the relations of the brain & nervous system in different stages of the animal scale, as affected by the causes of change which the theory supposes. The subject is difficult enough, particularly when reason begins to mix itself largely with instincts, forming itself an independent principle of selection, & therefore favourable to the theory; but not easy to deal with in detail

The differences in period of gestation, though made much of by many naturalists,⁸ do not really I think, make much against you They are probably easily convertible

Your large demands upon time, will be regarded & probably stated, by many, as an unfair method of dealing with objections, & vindicating transmutations otherwise inexplicable. I am very far indeed from making any such charge, knowing what I do of geological time, even under our present imperfect records But this is a point which somewhat needs to be kept by repetition before the mind of the reader—

I have put the foregoing comments down from recollection, & with but little method; & must bring this long & tedious letter to a close. Even