191 4- WhiTEHOUSE — The Natural History of Pla7tarm7ts. 45 
shews the same desire as that of the majority of animals 
to get right side up, and that quickly ; the creature twists 
itself, front end first, so as to bring the under side of the 
front end of the body next the surface on which it is moving ; 
it then proceeds forwards, gradually bringing more and 
more of the under surface in its normal position. 
The amateur experimentalist is frequently disappointed 
after repeating such apparently simple experiments and 
finds that all animals of the same species do not behave 
similarly when subjected to the same stimuli, and conse- 
quently he discredits the professional worker. But such 
experiments are by no means simple ; similar animals will 
only react similarly if they are in a similar physiological 
state ; the simplest way of illustrating this is by watching 
the hungry and the satisfied worm deal with food ; the 
former will greedily devour e\'en poor quality while 
latter may refuse a tit -bit. The determination of the 
physiological state of a worm is not a simple matter. 
In the same connexion the history of the individual is 
an important determinant of a creature's behaviour towards 
stimuli ; in other words, past experience has an important 
bearing on an animal's attitude towards things around it. 
Says Bergson, " the present moment of a living body does 
not find its explanation in the moment immediately before ; 
all the past of the organism must be added to that moment, 
its heredity — in fact the whole of a very long history." 
Let us take an illustration. Planar ia alpina is found in 
swiftly -flowing streams ; the animals are always found 
attached to the under surface of stones lying at the bottom ; 
I have never found them on wood, even if it be firmly 
fixed in the gravel or wedged between stones. Past ex- 
perience has no doubt taught this planarian that wooden 
things are liable to subject it to considerable buffetings, 
while stones at any rale are more to be relied on for an un- 
disturbed existence. 
Again, if Planaria alpina and Polycelis nigra (both fresh- 
water planarians) be placed in an aquarium, the former 
never, while the latter frequently ghdes along the under 
surface of the surface film of the water. The reason is 
