Thinks "mum" comes from shutting the mouth repeatedly as a sign of wanting to eat.
Down Beckenham Kent
It would be a great pleasure to me to give you any information, but I dare not trust my memory after so long an interval. In my little paper I have given only facts which were recorded at the time. My impression however is that ``mum'' arose from opening & shutting the mouth repeatedly as a sign of wanting to eat, for this movement makes a sound like the letter m. I feel sure that several of the consonants (m, b & d being excepted) were much more difficult for my children to utter correctly than the vowels. I can only repeat my regret that I cannot aid you in your very interesting investigation.
I remain, dear Sir | Yours faithfully | Charles Darwin