To J. S. Henslow   7 July [1855]

My dear Henslow

I write to thank you heartily for the seeds &c &c & all you have done.—

I do not think I have yet made it quite clear what I want marked in the Catalogue.¹ (but I am really ashamed to be so troublesome) it is not so much what you “think may turn out varieties” (to quote your own words) but what you think are really species, but yet are very closely allied to some other species: I well know how vague this is, & perhaps you will find it impossible to do; but certainly, judging from what I have seen in animals, one pretty often meets a pair or more real (as far as one can judge) species, which yet are far more closely allied together than the average.

Of course there is always the possibility & even sometimes probability of these close species turning out varieties.—

Ever most truly yours | C. Darwin

Perhaps you might (if you can do the job) mark the closely allied species in connection, thus in an imaginary genus

I sh^d. understand by this that you thought Q. pedunculata sessiliflora & humilis very closely allied species, but yet real species. I repeat perhaps the job will be impossible.—

P.S.² | If you think fit you can entirely leave out such genera, as Rubus, Salix &c, in which I suppose hardly anyone knows what a species is. By species I mean the ordinary rather vague acceptation of the term. If you pass over any genus entirely, just score out the generic name.

I think you once wrote on vibrios in wheat;³ I found the other day an Agrostis with every germen, (at least I opened a full dozen), with 1 or 2, or 3 little worms in them, & no stamens or stigmas.—

I am working at all varieties & have now got 46 kinds of Peas all growing together!