THH comments on G. A. Brullé's paper ["Researches upon the transformations of the appendages of the Articulata", Ann. & Mag. Nat. Hist. 13 (1844): 484–6].
14 Waverly Place,
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—
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 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— From the Antennules backwards the appendages in Mysis & 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 opthalmic 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 bruler'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 whose 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— If Brullés doctrine were correct therefore the Heart & Brain of 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 anything 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, 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 passage I have marked: he seems to me to say that, not the most highly complex, but the most characteristic organs are those 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—when the judicious builder begins with putting together the simple rafters— According to Brullés notion of Nature's operations 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” 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 Brullés 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
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. Il en résulte donc cette conséquence remarkable que les appendices se montrent d'autant plus tôt que leur structure doit être plus complexe. On trouve, en outre, dans ces développements divers une nouvelle preuve de l'analogie des appendices. Ainsi les pattes n'ont pas de transformation à subir elles ne se montrent que quand les autres appendices ont déjà revêtu la forme de mâchoires ou d'antennes. 3) Donc dans un animal articulé les appendices se montrent d'autant plus tard qu'ils ont moins de transformations à subir: c'est le complement de la loi précédente On peut par conseacute;quent juger du degré d'importance et de complication d'un appendice par l'époque même a laquelle il commence à se manifester pp. 282. 283—
- f1 2119.f1See the enclosure transcribed following the letter.
- f2 2119.f2T. H. Huxley 1856–7, 14: 639.
- f3 2119.f3Rathke 1840.
- f4 2119.f4Huxley's observations would, therefore, refute Gaspard Auguste Brullé's rule that the more essential and differentiated organs of an animal develop prior to those of less functional importance.
- f5 2119.f5See letter to T. H. Huxley, 5 July , n. 5. In the manuscript CD had sent to Huxley, he cited both Milne-Edwards 1844 and Milne-Edwards 1845 (see Natural selection, p. 303).
- f6 2119.f6CD had stated (Natural selection, p. 303): ‘he [Milne-Edwards] seems to think that according as the organs in question are most developed in any class, the earlier they appear in the embryo in that class’. Huxley marked the passage in the fair copy that CD had sent him (see letter to T. H. Huxley, 5 July , n. 2).
- f7 2119.f7The passage in question (Milne-Edwards 1845, p. 176) reads as follows:
Chez les Vertébrés, où l'appareil circulatoire doit acquérir une perfection très grande, et doit remplir un des rôles les plus importants, le cœur et les vaisseaux sanguins se forment, dès l'une des premières périodes de la vie embryonnaire, longtemps avant que le tube alimentaire se soit constitué, ou que le petit être en voie de formation ait acquis aucun des caractères propre aux animaux de sa classe.For CD's response to Huxley's point, see the letter to T. H. Huxley, 9 July .
- f8 2119.f8The quotation is taken from CD's manuscript (Natural selection, p. 303).
- f9 2119.f9Huxley copied out the text, with a few alterations and errors, from Brullé 1844, pp. 282–3.