Dear Professor Semper,

Since writing I have recalled some of the thoughts & conclusions which have passed through my mind of late years.¹ In North America, in going from N to S, or from E to W, it is clear that the changed conditions of life have modified the organisms in the different regions, so that they now form distinct races or even species. It is further clear that in isolated districts, however small, the inhabitants almost always get slightly modified; & how far this is due to the nature of the slightly different conditions to which they are exposed, & how far to mere interbreeding in the manner explained by Weismann, I can form no opinion.² The same difficulty occurred to me (as shown in my Var. of An. & Pl. under Dom) with respect to the aboriginal breeds of cattle, sheep &c in the separated districts of G^t Britain, & indeed throughout Europe.³ As our knowledge advances, very slight differences, considered by systematists as of no importance, in structure are continually found to be functionally important; & I have been especially struck with this fact in the case of plants to which my observations have of late years been confined. Therefore it seems to me rather rash to consider the slight differences between representative species, for instance those inhabiting the different islands of the same archipelago, as of no functional importance & as not in any way due to Natural Selection. With respect to all adapted structures, & these are innumerable, I cannot see how M. Wagner’s view throws any light.⁴ Nor indeed do I see at all more clearly than I did before from the numerous cases which he has brought forward, how & why it is that a long isolated form should almost always become slightly modified. I do not know whether you will care about hearing my further opinion on the point in question, for as before remarked I have not attended much of late years to such questions thinking it prudent now that I am growing old to work at easier subjects

Believe me yours very sincerely | Charles Darwin

I hope & trust that you will throw light on these points.—

P.S | I will add another remark, which I remember occurred to me when I first read M. Wagner.— When a species first arrives on a small island; it will probably increase rapidly & unless all the individuals change instantaneously (which is improbable in highest degree) the slowly more or less modifying offspring must intercross one with another & with their unmodified parents & any offspring not as yet modified. The case will then be like that of domesticated animals which have slowly become modified, either by the action of the external conditions, or by the process which I have called the unconscious selection by man.—i.e. in contrast with methodical Selection.⁵