In the former case--_i.e._ where allied species intermix in common areas--sometimes they intercross freely (e.g. _Primula vulgaris_ and _veris_, _Geum urbanum_ and _rivale_, _Rumex_, _Epilobium_, &c.), while in other instances they don't (e.g. _Ranunculus repens_ and _bulbosus_, _Lepidium Smithii_ and _campestre_, _Scrophularia nodosa_ and _aquatica_, &c.). Now, as regards the latter, I suppose you would not question that the 'physiological isolation' has to do with preventing the species from fusing? But, if so, by parity of reasoning, should we not expect to meet with _some degree_ of the same thing in the other cases, which, although not here sufficiently pronounced to block off frequent hybridisation, is nevertheless sufficient to prevent the species from blending over their common area? And here, I may say, I should not at all object to the charge of misunderstanding Darwin on any merely trivial point such as the one you mention. But in this instance it so happens that it is rather you who have misunderstood me. I know that 'a hybrid is not an intermediate form in his sense,' and this is just what constitutes my difficulty against his paragraphs quoted on p. 392 of my paper. For what I say is, these intermediate forms _ought_ to be hybrids, _unless physiological selection_, (i.e. _mutual sterility_) _has been at work_. 'In his sense' I cannot conceive how such 'intermediate forms' can exist in the circumstances described, seeing that they are _not_ hybrids, and yet that (in the absence of any hypothesis of physiological isolation for which I am contending) there is no reason given why the two interlocking species should not freely intercross. Regarding sexual selection I certainly am very much in earnest about its parallel to p.s.[62] If you intend the meaning of n.s. so as to embrace s.s. it will at the same time embrace also p.s. For s.s., like p.s., has nothing to do with life-preserving characters; yet, also like p.s., it has to do with the differentiation of specific forms. (There is no distinction to be drawn between 'the species of a cock' and 'the plumage of a cock': plumage is the most favourite part of a bird with ornithologists on which to found specific diagnoses.) Therefore, if p.s. is true at all--which, of course, is another question--even my celebrated powers of 'dialectical subtlety' are completely unable to perceive any difference between p.s. and s.s. in respect of their relation to n.s. Lastly, as regards Nägeli, no doubt he is an out-and-out Lamarckian, but I did not see that this should make any difference touching his opinion on a matter of fact not more connected with Lism. than Dism. I will look up 'Nature' for 1870. With best Christmas wishes and many thanks for botanical hints. December 26, 1888.