You ask how far I go in attributing organisms to a common descent; I answer I know not; the way in which I intend treating the subject, is to show (AS FAR AS I CAN) the facts and arguments for and against the common descent of the species of the same genus; and then show how far the same arguments tell for or against forms, more and more widely different: and when we come to forms of different orders and classes, there remain only some such arguments as those which can perhaps be deduced from similar rudimentary structures, and very soon not an argument is left.
[The following extract from a letter to Mr. Fox [October, 1855 (In this year he published ('Phil. Mag.' x.) a paper 'On the power of icebergs to make rectilinear uniformly-directed grooves across a submarine undulatory surface.'") gives a brief mention of the last meeting of the British Association which he attended:] "I really have no news: the only thing we have done for a long time, was to go to Glasgow; but the fatigue was to me more than it was worth, and E. caught a bad cold. On our return we stayed a single day at Shrewsbury, and enjoyed seeing the old place. I saw a little of Sir Philip (Sir P. Egerton was a neighbour of Mr. Fox.) (whom I liked much), and he asked me "why on earth I instigated you to rob his poultry-yard?' The meeting was a good one, and the Duke of Argyll spoke excellently."]