dition of -d is the process by which the present
form is rendered praeterite. The word _fell_ is formed from _fall_, by
changing a into e. The change of vowel is the process by which the present
form is rendered praeterite. Of the two processes the result is the same.
In what respect do they differ?

For the sake of illustration, let a new word be introduced into the
language. Let a praeterite tense of it be formed. This praeterite would be
formed, not by changing the vowel, but by adding -d. No _new_ verb ever
takes a strong praeterite. The like takes place with nouns. No _new_
substantive would form its plural, like _oxen_ or _geese_, by adding -en,
or by changing the vowel. It would rather, like _fathers_ and _horses_, add
the lene sibilant.

Now, the processes that change _fall_, _ox_ and _goose_ into _fell_,
_oxen_, and _geese_, inasmuch as they cease to operate on the language in
its present stage, are _obsolete_ processes; whilst those that change
_move_ into _moved_, and _horse_ into _horses_, operating on the language
in its present stage, are _vital_ processes.

A definition of the word _irregular_ might be so framed as to include all
words whose forms could not be accounted for by the vital processes. Such a
definition would make all the strong verbs irregular.

The very fact of so natural a class as that of the strong verbs being
reduced to the condition of irregulars, invalidates such a definition as
this.

s. 327. _Processes of necessity as opposed to processes of habit._--The
combinations -pd, -fd, -kd, -sd, and some others, are unpronounceable.
Hence words like _step_, _quaff_, _back_, _kiss_, &c., take after them the
sound of -t; _stept_, _quafft_, &c., being their praeterites, instead of
_stepd_, _quaffd_. Here the change from -d to -t is a matter of necessity.
It is not so with words like _weep_, and _wept_, &c. Here the change of
vowel is not necessary. _Weept_ might have been said if the habit of the
language had permitted.

A definition of the word _irregular_ might be so framed as to include all
words whose natural form was modified by any euphonic process whatever. In
this case _stept_ (modified by a process of necessity), and _wept_
(modified by a process of habit), would be equally irregular.

A less limited definition might account words regular as long as the
process by which they are deflected from their natural form was a process
of necessity. Those, however, which were modified by a