Holmes' Tropisms of Animals. 15 
scious choice of animals for certain kinds of stimulation. Graber, 
Sir John Lubbock and Paul Bert had studied the effect of colored 
lights upon animals and discovered that certain species congre¬ 
gated most abundantly under light of a certain color, while other 
species would gather in greater numbers under light of a different 
color. These aggregations were therefore considered as an index 
of the kind of color most pleasing to the aesthetic or other sensi¬ 
bilities of the animals. Similarly with the movements toward or 
away from lights. Earwigs and cockroaches were supposed to 
crawl away in secluded places because they like the dark and but¬ 
terflies were supposed to congregate in sunny spots because they 
enjoyed the sensation afforded by the sunshine. A somewhat more 
anthropomorphic interpretation of a tropism was suggested by 
Romanes in discussing why the moth flies into the flame of a 
candle. The conclusion arrived at was that the moth was drawn 
to the fatal flame out of curiosity, or the desire of investigating 
what manner of strange object a candle flame might be. 
The theory developed in Loeb’s Heliotropisms stands in a 
sharp contrast to the anthropomorphic views of his predecessors. 
Orientation of animals to light is supposed to take place in a more 
or less mechanical fashion like the orientation of plants. “Thest 
tropisms,” he says, “are identical for animals and plants. The 
explanation of them depends first upon the specific irritability of 
certain elements of the body surface, and second, upon the rela¬ 
tions of symmetry of the body. Symmetrical elements at the sur¬ 
face of the body have the same irritability; unsymmetrical ele¬ 
ments have a different irritability. Those nearer the oral pole 
possess an irritability greater than that of those near the aboral 
pole. These circumtances force an animal to orient itself toward 
a source of stimulation in such a way that symmetrical points on 
the surface of the body are stimulated equally. In this way the 
animals are led without will of their own either toward the source 
of the stimulus or away from it.” The moth flies into the flame, 
not out of curiosity or any other conscious motive, but simply 
because it cannot help it. 
In a very instructive series of experiments Loeb showed that 
heliotropism in animals obeys the same laws as heliotropism in 
plants. In both plants and animals it is the direction of the rays 
that controls the direction of movement. In both plants and ani- 
