(See _Nature_, vol.xxxix.p.

127.)] [Footnote 46: _Origin of Species,_ p.

72.] [Footnote 47: Darwin's latest expression of opinion on this question is interesting, since it shows that he was inclined to return to his earlier view of the general, or universal, utility of specific characters.

In a letter to Semper (30th Nov.

1878) he writes: "As our knowledge advances, very slight differences, considered by systematists as of no importance in structure, are continually found to be functionally important; and I have been especially struck with this fact in the case of plants, to which my observations have, of late years, been confined.