1891.] The Heltotropism of Hydra. 417 
which, as a rule, it is only the chlorophyll-containing forms that 
seek the light. 
The main purpose of the heliotropic movements, as I am 
convinced, is simply to place the animals in the position of 
maximum food supply, and the entire cycle of movements of 
which heliotropism is a factor may be explained on the same 
basis. The favorite and usual food of Hydra consists of various 
minute Crustacea,—Daphnia, Cypris, and other Entomostraca, 
especially the first named,—though it will readily devour insect 
larvæ and many other small animals. It is a well-known fact 
that Daphnia and related forms manifest ina high degree a helio- 
tropism of the same character as that of Hydra,—i. e., positive 
in moderate light, negative in strong light—and it must result 
from this that so far as the movements of the two animals are 
determined by light the tendency will be, in the long run, for the 
Hydras to collect in the localities most frequented by their prey. 
It is impossible to study an aquarium well stocked with the two 
animals without being struck by the immense advantage secured 
to the Hydras by their position on the illuminated side near the 
surface. In this region the Crustacea often swim in swarms, 
darting about through a forest of outstretched Hydras, many of 
which are gorged with food and actively budding, while in other 
parts of the aquarium both animals are far less abundant. The 
power of seeking the light, or of avoiding it when too strong, 
thus confers upon the blind, sluggish Hydra a means of pursuing 
and capturing its active and highly organized prey, and a vague, 
diffused sensibility to light becomes in this way of vital importance 
to its possessor, and may be brought under the action of natural 
selection. It cannot be doubted that individuals possessing a 
sensibility higher than the average will have a distinct advantage 
over the others, so that natural selection will tend to perpetuate 
them. An interesting feature of the case is that the increased 
food supply directly increases the rate of reproduction, —Ż. €., by 
budding,—so that, in the long run, individuals of high sensibility 
will multiply more rapidly than those of low sensibility, and leave 
a larger number of descendants in increasing proportion from 
generation to generation. It may be noted, further, that the 
