External Factors that Influence Growth 263 
The table appears to show that tadpoles grow faster in light, 
but it may be doubted whether this is due to the influence of the 
light on the tadpole itself rather than indirectly to the influence 
of light on the plants that grow in the water, which are used by 
the tadpole as a part of its food. Possibly also the setting free 
of oxygen by these plants during the daytime may make the 
conditions more favorable for growth. Too much caution cannot 
be used in considering all possible sides of the experiment. 
Yung also made experiments with other animals. Two hun- 
dred fertilized eggs of the trout were put into four liter vessels, 
and kept in running water. Those exposed to daylight hatched 
a day sooner than those in the dark. In this case the question of 
food does not enter into the problem, and the running water 
would seem to equalize other conditions. The eggs of the pond 
snail, Lymnza, hatched in the light in 27 days; in the dark in 
33 days. 
It has been known for some time that for plants the blue rays 
act more nearly like white light, while the yellow rays give more 
nearly the effect of darkness. Some work has been done with 
animals to test the effects of light of different colors. Yung 
finds that Hydra viridis dies if kept in the dark. It grows more 
rapidly in violet light than in green, and in green than in white, 
and in white than in red light. Since hydra contains green bodies 
supposed to be green algz, the influence of light is probably on 
the alge, and it is significant to find that the order of colors given 
is, in a general way, found to have the same effect on green plants; 
but it is doubtful whether these results can be accepted as final, 
because it is not evident that sufficient precautions were taken 
to have the intensity of the light the same in all cases and the 
food conditions also identical. 
Yung also carried out some experiments with tadpoles reared 
behind screens of nearly monochromatic colors (solutions). 
At the end of a month three tadpoles taken at random gave the 
following results: — 
