Dear Sir!

I need not tell You, how happy Your letter has made me.¹ If there was anything exciting my study, it was to change the manner of practical Zoology into that shape, which it must take after Your Origin of Species. There is a peculiarity in human spirit, that enables one to unite two quite different opinions. In theory people is of opinion, Natural Selection with all its Consequences is right,—and meanwhile in praxi ² they follow the old rule and feel not the inconsistency of such doing.

It was my endeavour to try, whether my favourite animals the Arthropoda would not allow a reformation by applying Your Principles. I have worked since two years nothing but Embryology,—and I see now, this was the right way.³

The little Paper, I sent You, will scarcely be able to give a true idea of what I stated, to anybody; it is too short and the matter too complicated for such brief communication.⁴ I am happy to say, that the first Volume of my larger work on the Morphology of the Arthropoda is almost ready to be printed. It contains the general foundations, and the special application to the Crustacea.⁵ It is wonderful with how great a surety the genealogical tribe is to be stated and how simply the morphological specialities are to be understood as soon as they are brought under the principle of Natural Selection.

I can tell You perhaps by some few words one of the most striking facts in Cirripeds. I cannot enter into the proves, but only give You the result and ask confidence for my investigations.

The rudiment of the dorsal Spine of Zoëa is to be found in all classes of the Crustacea,—except in the Copepoda, one of the freshest and youngest. Perhaps there it is overlooked; I myself never treated them in special. This proves that all Crustacea have passed through the Zoëa, and this is necessary for the declaration of a quantity of facts, which without such view never could be declared.⁶ You find the Rudiment even in Lepas, in the Cypris state.⁷ It is situated above the mandibles on the back, between the prehensile Antennae.

The same Rudiment You meet in Evadne Nordmanni;⁸ but when I first saw this remarkable creature, I was struck by the shape of it, for instead of seeing a rudimentary, functionless organ, I found an organ so well shaped, as ever an organ that has a distinct function to undergo. But I did not see any function. You can believe my joy, when I found in a small Paper of Professor Leuckart in the Archiv für Naturgeschichte, that this little organ, called by him “saugnapfartiges Haftorgan” was used by the little animal to fix its body to the glass, wherein it was observed.⁹ I now understood the not-rudimentary character.

This same thing You meet in Lepas. But soon there it becomes elongated and as long as the prehensile antennae, near which it is fastened to the body, on which the animal afterwards is to be found. It grows and grows, its muscular character is more and more developed, the ovaries are placed into it and finally it represents—the petiolum¹⁰ of the Cirripedes! Is’nt that most striking? Such a change of function?

What belongs to the Extremities of the Cirripeds, their close affinity to the Cladocera and Phyllop⁠⟨⁠o⁠⟩⁠da¹¹ enables us to follow another interpretation. The observations of Krohn, Mecznikow, and Pagenstecher state, that the first Antennae bear the Cement Apparatus,—and I think, they are right.¹² What is to be observed in the prehensile antennae might be the Schalendrüse,—the homologous organ of the Grüne Drüse and similar organs.¹³ The second Antennae and the Mandibles, the remaining Nauplius-Extremities are lost. Now we have a labrum and three pairs of maxillae; in Your nomenclature mandible, inner maxilla and outer maxilla.¹⁴ What You describe as Palpus is, I think, the underlip or tongue (Savigny) that little bifid organ, which we meet in every Arthropodous animal opposite to the Labrum as the hinder wall of the mouth.¹⁵

Thus all is in agreement. And surely, I would never have found but by applying genealogical ideas, and I cannot tell how strongly even at every step I am indebted to Your leading ideas, that bring a splendid light into the Arthropoda-Confusion.

I’ll not enter into other chapters. I only will promise, that the first copy of my book will be forwarded to You; and surely, it will be my greatest pride, if You could say me, that there is something valuable in it and if You acknowledge my leading ideas as those Principles that Your book has impregnated on every free young spirit.

I know, dear Sir, You don’t like Compliments—and I dare say I cannot make them, remembering that word of Shakespeare: “And what they call compliments is like the encounter of two dog-apes”.¹⁶ So I am sure You’ll make a difference between Compliments and deep Veneration, which flows out of the heart of a young ardent champion for truth and Liberty. Pardon my words, but I feel it a duty to tell You them. Enthusiasm is one of the most beautiful privileges of Youth and it is very often the orgin of good and lasting things. You may therefore imagine, that it was a great desire of mine to see and to speak You, when I was last summer in England. But I dared not trouble You, knowing that Your health is not so strong as we all might wish.¹⁷ The more I owe You my greatest thanks, that You have sent me that letter, which will give me, as I might call it my scientific knighthood. I thank You, dear Sir!

Yours ever truly devoted | Anton Dohrn

Professor Haeckel sends his most sincere regards, and expresses with me the best wishes for Your health.