388 Evolution and Adaptation 
that of butterflies, for the light of the evening to which the 
moth reacts is less than the minimal to which the butterfly 
responds. 
Moths appear to pass into a sort of sleep during the day, 
while butterflies are quiescent only at night. The periodicity 
of the sleeping time continues, at least for several days, 
when the insects are kept in the dark. For instance, moths 
kept in the dark become restless as the evening approaches, 
as Réaumur observed long ago. It has been found in plants 
that this sort of periodicity may continue for several days, 
but gradually disappears if the plants are kept in the dark. 
By using artificial light, and exposing the plants to it during 
the night, and putting them in the dark during the day, a new 
periodicity, alternating with the former one, may be induced ; 
and this will continue for some days if the plants are then 
kept continually in the dark. 
Loeb tried the experiment of exposing the quiescent moths 
suddenly to a lower intensity of light, in order to see if they 
would respond equally well at any time of day. It was found 
that if the change was made in the forenoon, between six 
o'clock and noon, it was not possible to awaken the moths by 
a sudden decrease in the intensity of the light. But it was 
possible to do so in the afternoon, long before the appearance 
of dusk. It appears, therefore, that in this species, Sphinx 
euphorbi@, it is possible to influence the period of awaken- 
ing by decreasing the intensity of light, but this can be done 
only near the natural period of awakening. It seems to me 
that this awaking of a positively heliotropic animal by decreas- 
ing the light needs to be further investigated. 
The day butterflies are also positively heliotropic. Butter- 
flies of the species Papilio machaon, that have been raised 
from the pupa, remain quietly on the window in the diffuse 
daylight of a bright day. They can be carried around on 
the finger without leaving it, but the moment they come into 
the direct rays of the sun they fly away. 
