My dear Sir

I am in truth ashamed to trouble you so soon again; but since I sent you the list of Silene, I find that I very stupidly overlooked a list of Cucubalus, which I had forgotten that Gærtner ranked as quite distinct genus.—²

I have now copied a list in pairs of the crosses which he made in Cucubalus, & it would be a very great kindness, if you would look it over & mark whether in each pair, you consider the male & female as only varieties of the same species or as distinct species, & allow me to state that such is your opinion.—³

My object is to show that where the fertility of a cross makes a very close approach to perfect fertility there is often difference of opinion whether the forms are varieties or species. If any of the pairs are undoubtedly good & distinct species, the case does not concern me. If this would not cause you much trouble, it w^d be a great kindness, for I have already got a curious parallel list of graduated evidence from fertility & ordinary evidence on what to call species and what varieties.—

My dear Sir | Yours very sincerely | C. Darwin