You obviously failed to spot in my message that James died at the same place as his mother, 6 Salisbury Place, Edinburgh. Like you, I haven't found any record of his birth but his death certificate proves that he was a son of Thomas Downie and Margaret Dudgeon:
1858 Deaths Edinburgh 685/5 no 151
James Downie .... Single
died 1858 April Nineteeth ... 6 Salisbury St Edinburgh male age 32 years
Father Thomas Downie Gardener deceased
Mother Margaret Dudgeon
cause of death Disease of Heart
buried at Grange Cemetery
informant Charles Downie brother
This is only a partial transcription from a Downie friend who has a seat in New Register House and sends me all the Downies she finds. He was probably born in Moffat in about 1825 and could have been a twin of Jane, but I haven't found him in the 1851 Census so far - I rely mostly on FreeCEN. I don't know whether my friend missed out James's occupation or whether it was not on the certificate.
I have a general interest in all Downies since that is my surname. John is a relatively famous Downie, having a street named after him (Downie Terrace, Corstorphine, where he died on 5 November 1892). I have a website at
http://www.downieabz.org.uk where I have information about my own family plus a number of more famous Downies. There is room for an entry on John Downie!
There is an outside chance that this family is related to mine since my ancestors moved from the East of Fife to the Edinburgh area and then Juniper Green, and there are some gardeners among them, including my GG Grandfather David Downie, born 10 March 1821 at Penicuik. Thomas is not a name which appears in earlier generations of the family but David had a brother Thomas born 12 October 1822 at Currie and there were several Thomas Downies in later generations.
I would be very interested to know what further information you have on Thomas Downie, John's father as I would like to be able to connect him to other Downie families I have in my database. I have John's obituary from The Scotsman but it gives no details of his origins - I hope you can give me a transcript (and the source) of the obituary you have, plus the source of your information about the date and place of his death (if different).
I'd be glad to exchange information with you about this family and will PM you my email address.
Ian