CHAPTER VII LAWS OF VARIATION: VARIETIES & SPECIES COMPARED. INTRODUCTION The writing dates for chapter VII are not clear from Darwin's Pocket Diary, which lumps it together with the following chapter. Having finished chapter VI at the end of March, 1857, Darwin presumably commenced writing chapter VII at the beginning of April; for, to judge from the identical four illustrative facts appearing both in a paragraph in Darwin's letter of April 8, 1857, to 1 Hooker and on folio 8 of chapter VII, it seems likely that both passages were written at nearly the same date. Probably by June 5 Darwin's writing had almost reached folio 105, for on that date he mentioned in a letter to Hooker2 that: 'I have been so much interested this morning in comparing all my notes on the variation of the several species of the genus Equus and the results of their crossing.' This topic, the next to the last discussed in the chapter, forms the subject of folios 105 to 113, and Darwin presumably started to write it up soon after he had reviewed his relevant notes. Darwin had probably completed chapter vII by July 5,1857, when he mailed to T. H. Huxley a fair copy of folios 41 to 44 together with the following letter: Down, Bromley, Kent July 5 [1857] My dear Huxley Will you be so kind as to read the two enclosed pages as you said you would, and consider the little point therein referred to. I have not thought it worth troubling you with how far and in which way the case concerns my work, the point being how far there is any truth in MM Brullé and Barneoud. My plan of work is just to compare partial generalisations of various authors and see how far they corroborate each other. Especially I want your opinion how far you think I am right in bringing in Milne Edwards view of classification. I was long ago much struck with the principle referred to: but I could then see no rational explanation why affinities should go with the more or less early branching off from a common embryonic form. But if MM Brullé and Barneoud are right, it seems to me we get some light on Milne Edwards views of classification; and this particularly interests me. I wish I could anyhow test M. Brullé's doctrine; as in Vertebrates the head 1 2 ML, no. 56. L & L, II, 101-2; NY, I, 459. 275 LAWS OF VARIATION consists of greatly altered Vertebrae, according to this rule, in an early part of the embryonic development of a Vertebrate animal, the head ought to have arrived more nearly to its perfect state, than a dorsal or cervical vertebra to its perfect state. How is this? I have been reading Goodsir, but have found no light on my particular point. The paper impresses me with a high idea of his judgment and knowledge, though, of course, I can form no independent judgment of the truth of his doctrines. But by Jove it would require a wonderful amount of evidence to make one believe that the head of an elephant or tapir had more vertebrae in it, than the head of a Horse or Ox. Many thanks for your last Lecture. How curious the development of Mysis! yours very sincerely C H. DARWIN Do you know whether the embryology of a Bat has ever been 1 worked out? 14 Waverley Place, July 7, 1857 My dear Darwin— I have been looking into Brullé's paper, and all the evidence I can find for his generalization (adduced by himself) is contained in the extract which I inclose—Let us dispose of this first— Paragraph No. 1. is true but does not necessarily either support or weaken his view, which rests on paragraph No. 2.—Now this paragraph is a mass of errors—You will find in my account of the development of Mysis that the antennae appear before the gnathites are any of them discoverable—& Rathke states the same thing with regard to Astacus—and I believe it to be true of Crustacea in general. The second statement, that the legs do not appear until the buccal appendages have taken on their adult form is equally opposed to my own observations & to ‹all› those of all who have worked in this field. It would have been very wonderful to me to find Brullé resting such a generalization on such a basis, even had his two affirmations as to matter of fact, been correct. But as they are both wrong— one can only stand on one's head in the spirit— Next as to the converse proposition marked 3). It is equally untrue—Mouths antennules backwards The appendages in Mysis 1 CD. MSS. box 145, pp. 152. W. R. Dawson, ed., The Huxley Papers a descriptive catalogue.. .(London, 1946), p. 26, lists this as Darwin letter no. 22 and gives the date July 5, 1857. 276 LAWS OF VARIATION & in Astacus appear in regular order from before backwards wholly without respect to their future simplicity or complexity— and, what is still worse for M. Brullé, the ophthalmic peduncles, which as you know well are the most rudimentary & simple of all the appendages in the adult make their appearance at the most very little later than the mandibles & increase in size at first out of all proportion to the other appendages M. Brullé bases his whole generalization upon what he supposes to occur in the Crustacea—whereas the development of both Astacus & Mysis—affords the most striking refutation of his views Tant pis pour Brullé'! And now having brûler'd Brullé (couldn't help the pun) I must say that I can find no support for his generalization elsewhere— There are two organs in the Vertebrata where developmental history is especially well qualified to test it—the Heart & the Nervous system—both presenting the greatest possible amount of variation in their degree of perfection in different members of the vertebrate series—The heart of a Fish is very simple as compared with that of a Mammal & a like relation obtains between the brains of the two—[Darwin's comment: 'Good'.] If Brullé's doctrine were correct therefore the Heart & Brain of the Fish should appear at a later period relatively to the other organs than those of the Mammal—I do not know that there is the least evidence of any- thing of the kind—On the contrary the history of development in the Fish & in the Mammal shews that in both the relative time of appearance of these organs is the same or at any rate the difference if such exist is so insignificant as to have escaped notice— With regard to Milne Edwards views—I do not think they at all involve or bear out Brullé's. Milne Edwards says nothing, [CD.: '?See to this'.] as far as I am aware about the relative time of appearance of more or less complex organs—I should not understand Milne Edwards doctrine as you put it, in the paragraph I have marked: he seems to me to say that, not the most highly complex, but the most characteristic organs are the first developed —Thus the chorda dorsalis of vertebrates—a structure characteristic of the group but which is & remains excessively simple, is one of the earliest developed—The animal body is built up like a House —where the Judicious builder begins with putting together the simple rafters—According to Brullé's notion of Nature's operation he would begin with the cornices, cupboards, & grand piano. It is quite true that "the more widely two animals differ from one another the earlier does their embryonic resemblance cease" 277 LAWS OF VARIATION but you must remember that the differentiation which takes place is the result not so much of the development of new parts as of the modification of parts already existing and common to both of the divergent types.— I should be quite inclined to believe that a more complex part requires a longer time for its development than a simple one; but it does not at all follow that it should appear relatively earlier than the simple part. The Brain, I doubt not, requires a longer time for its development than the spinal cord. Nevertheless they both appear together as a continuous whole, the Brain continuing to change after the spinal cord has attained its perfect form. The period at which an organ appears therefore, seems to me not to furnish the least indication as to the time which is required for that organ to become perfect You see my verdict would be that Brullé's doctrine is quite unsupported—nay is contradicted by development—so far as animals are concerned—& I suspect a Botanist would give you the same opinion with regard to plants— Ever yours faithfully T. H. HUXLEY [Passage copied by Huxley from Brullé, Ann. sci. nat. ser. 3 zool. 2 (1844), 282-3:] 1) En suivant, comme on l'a fait dans ces derniers temps les phases du developpement des Crustacés, on voit que les pièces de la bouche et des antennes se manifestent avant les pattes; celles ci ne se montrent que par suite des developpements ultérieurs —2) De leur côté, les antennes sont encore fort peu développées que les pièces de la bouche le sont déjà plus; enfin c'est lorsque les appendices buccaux ont revêtu la forme qu'ils doivent conserver que les pattes commencent à paraître. II en résulte donc cette conséquence remarkable [sic], que les appendices se montrent d'autant plus tôt que leur structure doit être plus complexe.