again later in the day, or perhaps he would make a move-
ment to try it, and then, as we should say of a human
being, recollect himself. More marked cases of this gradual
growth and equally gradual obliteration of the effects of
experience may be seen in other animals. Thus Dahl ¹
gave a spider a fly dipped in turpentine. The spider
sprang upon it three times, and, disliking the turpentine,
withdrew on each occasion. After the third attempt it
turned aside when the fly came in sight. Three instances
sufficed to inhibit the impulse to spring. Nor would the
spider for some hours attack a fly of the same species that
was innocent of turpentine. The next day the spider
attacked the fly in turpentine again, but once only.

Here three experiences were necessary to establish the
result in the first instance, and on the following day the
effect was partially obliterated, so that one instance was
needed to renew it. Every one is familiar with similar
points in the working of human memory--the "fixing" of
a verse by repetition, its fading with time, and its revival
by perhaps a single reading.

It may be worth while to place in contrast with the in-
stances given above a case of extreme stupidity in this
respect. Speaking of a number of fish in an aquarium,
Mr. Bateson says:--

"None of the fish seem to get any lasting appreciation of the
nature of the plate glass wall of the tank. The same fish will
again and again knock its head against the glass in trying to
seize objects moving on the other side."

A dangling button, or even a curl of smoke, may attract
them. After repeated attempts they will desist, but some
of the oldest inhabitants, which have been living in the
aquarium about a year, will try again next time. This
failure is no doubt partly due to dullness of sense per-
ception, but, considering the length of time allowed for
learning, it implies a surprising incapacity for turning ex-
periences to account--a stupidity surpassing even that of
Möbius's pike, which, after dashing itself for three months
against a glass partition in the attempt to get at some
minnows, became at last so firmly persuaded of the danger

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