My dear Sir

Thank you for your very kind letter. It is a great satisfaction to me to find that my views gain ground with you.¹ If I have really found the secret, I shall owe my success to your writings which have given the key to the whole book of animal and vegetable life.²

Some acacia-twigs which I have been examining have a very curious disposition of their leaves, unintelligible to me except as a distortion of an original 2-ranked order. Let me copy the records of a few specimens from my notes:—

Fig. I shows the order usually found in the acacia, viz. 5 nearly vertical (in my diagram, nearly radial) ranks:—phyllotactic order, nearly $\frac{2}{5}$.

Fig. II shows the same 5-ranked order prevailing in the first 23 members, but disturbed by a dislocation of 24 and 25. If we join by a dotted line all the even N^os. (which are the successive members of one of the supposed original two ranks), and by another dotted line all the odd n^os. (which make the other rank), we see in the last two leaves (24 & 25) the beginning of a reverse twist.

Fig. III shows a series of reverse twists, alternately to right and left, with 2, or 3, members in each twist. But take away the dotted line, and all is confusion.

In Fig. IV the zigzags contain alternately 1 and 2 members, and are so regular that they produce radial ranks with an evident cycle of 6 leaves between successive members of each rank. (The series 1, 5, 7, 11, 13, 17, 19, 23, 25 has to be resolved into two, 1, 7, 13, 19, 25, and 5, 11, 17, 23; and these two with the remaining four evident ranks give a system of 6 ranks in which the 6-cycle prevails.) But a 6-cycle is at variance with any regular spiral arrangement. The only evident interpretation of this most curious phenomenon is, that we have here the result of a zigzag, plicate condensation of two originally vertical ranks. This is just what we might have expected à priori. I had previously noticed the same kind of behaviour in my imitative mechanism.³ When I prevented it from twisting in a uniform spiral, it effected its condensation by a zigzag corrugation of each of the two vertical ranks. In the diagram it may be noticed that all the even n^os. lie to one side, and all the odds to the other.

Fig. V exhibits a simpler form of zigzag, with one member in each ‘lap’ (except the lap containing n^o. 9, where there are two). This specimen seemed at first sight to have its two ranks accurately vertical, but close examination showed that they were in zigzag. From this to perfect accuracy the transition would be easy; but I have not yet found a specimen of acacia-twig with perfectly vertical ranks, like those of elm.

I may remark that, in making these diagrammatic records, I used my best care to determine the relative positions of the leaves, near and far, by tracing the ridges and furrows of the bark; and it was only towards the end of the series that I had the zigzag interpretation forced upon me by a discussion of the records already obtained.

I think you will be pleased with this new piece of evidence, and will agree with me that a theory which will account for these anomalous varieties, as well as for the normal types, and bring them all into harmony with one another, has a good claim to acceptance.— At the same time I must apologize for thus taking advantage of your kindness, when I know your time is so fully occupied with other subjects.

Thank you very much for allowing me to look forward to an early visit to you—⁴ I am just about to make another change of abode, but shall not allow it to interfere with your invitation. Let me repeat that I am most anxious my call should not be troublesome   I am much obliged to you for the notice of Keoner’s(?) Alpine observations.⁵

Believe me, my dear Sir, | With great respect. Yours very sincerely | Hubert Airy

Charles Darwin Esq^r. M.A., F.R.S. &c &c