agent of transmission or handling instrument, whom we are
comparing to the brain, might be so awkward, slow, and inefficient as a
go-between that the possible ability of Brown and Smith in passing would
be nullified or greatly hampered. But if the inefficiency of Jones is
blamable to his inexperience, it evidently can be changed to efficiency
by _sufficient right exercise_ in passing. The more of that sort of work
he does, in either direction, the better passer will Jones become.

His exercise, however, must be _in passing_ things, if _passing_
capability is to be developed. He would not become a better and quicker
_passer_ by any amount of exercise in taking things apart, or in
inspecting things--wholly dissimilar functions.

[Sidenote: Training in Passing]

Moreover, Jones would not become an expert passer of _glassware_ as a
result of practice in passing _bricks_, for the two kinds of things are
not handled alike. Indeed, the man accustomed to passing bricks might be
more likely to break glassware than another man who previously had no
particular skill in passing anything. The expert brick-passer would be
apt to forget sometimes that he was passing glass. His muscles might
treat the fragile ware with the rough habit acquired in passing bricks.

Plainly, discriminative-restrictive methods of training are required to
perfect capability in any _particular_ kind of physical passing; however
much skill in _general_ passing may have been developed. If Jones should
become expert in passing pails of liquid, he would nevertheless need to
train himself anew in order to pass frozen liquid efficiently in the
form of cakes of ice. And, to particularize still more, it would be
necessary for him to learn how to pass different liquids. Water and
thick molasses in pails should not be handled alike.

Similarly the various brain centers, as passers of different sense
impressions and mental reflexes in and out, require, each of them--like
Jones--the _specific_ exercises that will develop _their