The wonder, then, is not that it
took on a number of external morphological features, mere accretions on
its concrete inventory, but that, exposed as it was to remolding
influences, it remained so true to its own type and historic drift. The
experience gained from the study of the English language is strengthened
by all that we know of documented linguistic history. Nowhere do we find
any but superficial morphological interinfluencings. We may infer one of
several things from this:--That a really serious morphological influence
is not, perhaps, impossible, but that its operation is so slow that it
has hardly ever had the chance to incorporate itself in the relatively
small portion of linguistic history that lies open to inspection; or
that there are certain favorable conditions that make for profound
morphological disturbances from without, say a peculiar instability of
linguistic type or an unusual degree of cultural contact, conditions
that do not happen to be realized in our documentary material; or,
finally, that we have not the right to assume that a language may easily
exert a remolding morphological influence on another.

Meanwhile we are confronted by the baffling fact that important traits
of morphology are frequently found distributed among widely differing
languages within a large area, so widely differing, indeed, that it is
customary to consider them genetically unrelated. Sometimes we may
suspect that the resemblance is due to a mere convergence, that a
similar morphological feature has grown up independently in unrelated
languages. Yet certain morphological distributions are too specific in
character to be so lightly dismissed. There must be some historical
factor to account for them. Now it should be remembered that the concept
of a "linguistic stock" is never definitive[173] in an exclusive sense.
We can only say, with reasonable certainty, that such and such languages
are descended from a common source, but we cannot say that such and such
other languages are not genetically related. All we can do is to say
that the evidence for relationship is not cumulative enough to make the
inference of common origin absolutely necessary. May it not be, then,
that many instances of morphological similarity between divergent
languages of a restricted area are merely the last vestiges of a
community of type and phonetic substance that the destructive work of
diverging drifts has now made unrecognizable? There is prob