Just a few comments about "subtratum
influences":
(1) Really we know poorly many of the languages
that were at one substratum of Latin and most of the suposed "substratum
influences" are only documented in popular inscriptions many time after the
virtual extinction of such languages.
(2) In other context, for example that of the split
of Spanish varieties in America, we know very well the substrate but only a
small portion of the features can be atributed to the substratum. (For example
my native tonge is Peninsular Spanish and know Nahuatl and Mexico Spanish, and
it is evident that only just a few features that distinguish Mexico Spanish
from Peninsular Spanish are atribuable to the Nahuatl subtratum. We can improve
the comparation takin into account only Mexico Spanish, Old Medieval Spanish and
Nahuatl and we observe that the innovations of Mexico Spanish are highly
independent of the Nahuatl substratum [and other possible substrata that I only
know partially]).
But despite (1) & (2) the "substratum
influence" is currently mentioned as one of the main causes of language
divergence!
Davius S.