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—¹ Let us dispose of this first— Paragraph N^o. 1. is true but does not necessarily either support or weaken his view, which rests on paragraph N^o. 2.— Now this paragraph is a mass of Errors.— You will find in my account of the development of Mysis that the antennæ appear before the gnathites are any of them discoverable² — & Rathke states the same thing with regard to Astacus ³ —and I believe it to be true of Crustacea in general.⁴

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.⁵ 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:⁶ he seems to me to say that, not the most highly complex, but the most characteristic organs are those first developed—⁷ 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”⁸ 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