not transmit any effect, but do so if pressed by a weight of 1/78000 grain
("Insectivorous Plants" 263). On the other hand, the filament of Dionoea
may be quietly loaded with a much greater weight, while a touch by a hair
causes the lobes to close instantly. This has always seemed to me a
marvellous fact. Thirdly, I have been accustomed to look at the coming in
of the sense of pleasure and pain as one of the most important steps in the
development of mind, and I should think it ought to be prominent in your
table. The sort of progress which I have imagined is that a stimulus
produced some effect at the point affected, and that the effect radiated at
first in all directions, and then that certain definite advantageous lines
of transmission were acquired, inducing definite reaction in certain lines.
Such transmission afterwards became associated in some unknown way with
pleasure or pain. These sensations led at first to all sorts of violent
action, such as the wriggling of a worm, which was of some use. All the
organs of sense would be at the same time excited. Afterwards definite
lines of action would be found to be the most useful, and so would be
practised. But it is of no use my giving you my crude notions.


LETTER 421. TO S. TOLVER PRESTON.
Down, May 22nd, 1880.

(421/1. Mr. Preston wrote (May 20th, 1880) to the effect that
"self-interest as a motive for conduct is a thing to be commended--and it
certainly [is] I think...the only conceivable rational motive of conduct:
and always is the tacitly recognised motive in all rational actions." Mr.
Preston does not, of course, commend selfishness, which is not true
self-interest.

There seem to be two ways of looking at the case given by Darwin. The man