HELIOTROPISM OF ANIMALS 7 
in all cases, it would prove that the protoplasm of a multi- 
cellular plant behaves just like the naked, creeping plas- 
modium, which is also heliotropically irritable. 
III]. SUMMARY OF THE MECHANICAL EFFECTS OF LIGHT IN THE 
ANIMAL KINGDOM WHICH ARE THUS FAR KNOWN 
I shall in this chapter summarize briefly the facts and 
views in regard to the movements of animals under the in- 
fluence of light, so far as they are known up to the present 
time. These may be divided into three groups: 
1. Casual observations of the older authors (Réaumur, 
Trembley). These are unprejudiced records of simple obser- 
vations. 
2. Modern investigations on the effects of light from an 
anthropomorphic standpoint. The movements of animals 
are not attributed to mechanical causes, but to supposed 
human sensations of the animals. 
3. Investigations according to the method of Sachs, which, 
however, have been applied only to Protozoa. The last- 
named observations are the most important in these three 
groups. 
The earliest account of the effects of light on animals 
which I have found in the literature is by Réaumur.’ He 
found that moths which are attracted by the candle flame “do 
not fly from flower to flower during the day.” Since he saw 
chiefly the males fly into the flame, he raised the question 
as to whether or not the female moths emit light like glow- 
worms. ‘Do not the females of the nocturnal Lepidoptera 
emit a light too feeble to make an impression on our eyes, 
but sufficiently strong to act on those of their males?” He 
had observed, evidently, that the males of the glow-worm 
which are attracted by the light to the aboral end of the 
females likewise fly into the light. Réaumur was, moreover, 
1RBauMuR, Mémoires pour servir & Uhistoire des insectes, Vol. I, 1, p.330 (Amster- 
dam, 1748). 
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