  The words of a language are randomly replaced in time by new ones, but it has
long been known that words corresponding to some items (meanings) are less
frequently replaced than others. Usually, the rate of replacement for a given
item is not directly observable, but it is inferred by the estimated stability
which, on the contrary, is observable. This idea goes back a long way in the
lexicostatistical literature, nevertheless nothing ensures that it gives the
correct answer. The family of Romance languages allows for a direct test of the
estimated stabilities against the replacement rates since the proto-language
(Latin) is known and the replacement rates can be explicitly computed. The
output of the test is threefold:first, we prove that the standard approach
which tries to infer the replacement rates trough the estimated stabilities is
sound; second, we are able to rewrite the fundamental formula of
Glottochronology for a non universal replacement rate (a rate which depends on
the item); third, we give indisputable evidence that the stability ranking is
far from being the same for different families of languages. This last result
is also supported by comparison with the Malagasy family of dialects. As a side
result we also provide some evidence that Vulgar Latin and not Late Classical
Latin is at the root of modern Romance languages.
