HELIOTROPISM OF ANIMALS 



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 (Reaumur, 

 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 Reaumur. 1 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. Reaumur was, moreover, 



i REAUMUH, :?' moires pour servir d, Vhistoire des insectes, Vol. I, 1, p. 330 (Amster- 



d.-. -.1748). 



