[Footnote 6: Carpenter's _Principles of Comparative Physiology_, pp.
616-17.]
[Footnote 7: With the exception, perhaps, of the Myxinoid fishes, in
which what is considered as the nasal orifice is single, and on the
median line. But seeing how unusual is the position of this orifice, it
seems questionable whether it is the true homologue of the nostrils.]
[Footnote 8: In the _Westminster Review_ for April, 1857; and now
reprinted in this volume.]
[Footnote 9: See Essay on "Progress: its Law and Cause."]
[Footnote 10: This was written before the publication of the _Origin of
Species_. I leave it standing because it shows the stage of thought then
arrived at.]