The intervals between baptisms are rather irregular and some very short, e.g. 10 Aug 1589 to 8 May 1590 is only 9 months. [I'm not certain whether old style, with the year ending in March, is being used here, as it would have been in the original register, but for this example it doesn't matter.]
So perhaps there were two overlapping sequences of births with different John Martyn fathers.
The mention of John Martyn & Elyzabeth Nankevall, m St Columb Min 26 Apr 1579, reminds me they come into the putative ancestry of the Thomas Martyn (1668-1734) with many descendants; see:
http://wc.rootsweb.ancestry.com/cgi-bin/igm.cgi?op=PED&db=:1501551&id=I8This tree gives Thomas's father Richard as a son of John & Elizabeth, but there seems too long a time gap. I've just looked at the Cornwall OPCs online, and a more plausible line would insert an intermediate generation, as follows.
Thomas, c 20 Feb 1580, son of John Martyn and possibly Elyzabeth Nankevall, could be the father of seven children christened 1607-1624 in St Columb Min, of whom the fifth is Richard, c 25 Apr 1619. He'd be a little old to be father of Thomas, but not impossibly so (m 1668, Margaret Mundy b 1644). Further he is quite likely the Richard Martyn Senr. buried 26 Dec 1693 St Columb Min.
["Senior" as opposed to another Richard Martyn buried 29 June 1693.]
This is all rather speculative - can anyone disprove or support?
Denis