They are (1) the universality and large amount of normal
variability, (2) the extreme rigour of Natural Selection, and (3) that
there is no adequate evidence for, and very much against, the
inheritance of acquired characters.
It was only some years later, when I began to write on the subject and
had to think out the exact mode of action of Natural Selection, that I
myself arrived at (1) and (2), and have ever since dwelt upon them--in
season and out of season, as many will think--as being absolutely
essential to a comprehension of organic evolution. The third I did not
realise till I read Weismann, I have never seen the sufficiency of
normal variability for the modification of species more strongly or
better put than in your letters to Bates. Darwin himself never realised
it, and consequently played into the hands of the "discontinuous
variation" and "mutation" men, by so continually saying "_if_ they
vary"--"without variation Natural Selection can do nothing," etc.
Your argument that variations are not caused by change of environment is
equally forcible and convincing. Has anybody answered de Vries yet?
F. Darwin lent me Prof. Hubrecht's review from the _Popular Science
Monthly_, in which he claims that de Vries has proved that new species
have always been produced from "mutations," never through normal
variability, and that Darwin latterly agreed with him! This is to me
amazing! The Americans too accept de Vries as a second Darwin!--Yours
very sincerely,
ALFRED E.
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