This is one of those complrx issue which no one has answered properly.
Eliot opined that during the formation of Sundaland & the subsequent separation of the landmasses several taxa were established thus.
taxon newboldi Isthmus of Kra.
taxon humei Malaya.
taxon moerus Sumatra
taxon selinuntius Borneo.
taxon horsfieldi Java.
When Malaya rejoined the mainland, taxon newboldi moves into it. Thus two taxa humei & newboldi occur in Malaya.
The Javan taxon was shown to be different enough to elevated as a separate species Y. horsfieldii.
Eliot surmise the two taxa newboldi & humei have not fully intergrated & can be recognised as separate species.
He concluded that humei, the Sundanian form can be treated as a subspecies of the Javanese Y. horsfieldii.
Taxon newboldi would remain as a ssp. of the continental Y. baldus.
He suggest that the Sumatran moerus & the Bornean selinuntius are part of (ie ssp) Y. horsfieldii.
Many authors have not adopt this view.
Yukata does not recognise Y. horsfieldii humei & lump all under Y. baldus newboldi. http://yutaka.it-n.jp/sat/41000010.html
A DNA barcoding study down in 2015 by a team from the University of Malaya concluded that 1, taxon newboldi should be treated as a new species from Y. baldus and 2 Y. horsfieldi humei and Y. nebulosa are the same species.
See pdf article. https://www.google.com/search?q=ypth...QHnIUgXPtVQ5sM
These views are also not universally accepted.
It is easy to cross the border & mis-identifed something as Y. nebulosa. http://yutaka.it-n.jp/sat/41020001.html
As for taxon newboldi it probably can be defined as a separate species from Y. baldus.
However, we are only concerned about the differences of these two taxa Y. horsfieldi humei & Y. baldus newboldi .
Undoubtedly there will be some hybrids causing great difficulty.
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