1891.] The Heliotropism of Hydra. 421 
the case seems to me an interesting one, as illustrating both the 
correlations between associated organisms, and the nature of the 
conditions that may enable natural selection to operate at or near 
the beginning of a series of physiological and morphological 
modifications. 
IV. Color Discrimination —Like many other heliotropic forms, 
Hydra is chiefly affected by the blue rays. If strips of glass of 
various colors be fastened to the illuminated side of an aquarium, 
both species of Hydra show a very marked tendency to collect 
under the blue, and an equally marked avoidance of the red, green, 
yellow, or any combination of colors containing no blue. This 
preference for the blue is (within rather wide limits) independent 
of intensity. This is strikingly shown by the comparison of a 
light “ yellow” glass with a dark blue cobalt glass,’ the former 
being of high, the latter of low, intensity. If equal areas on the 
light side of an aquarium be covered (see Table IL.) (a) with 
yellow, (4) with blue, (c) with an opaque screen, anda fourth area 
(d) be left uncovered, the result is invariably that in the course of a 
few days the greatest number of Hydras will be found under the 
blue (allowance being of course made for the initial differences) ; 
the uncovered area stands next, and the shaded and yellow areas 
contain fewest, with no constant difference between them. That 
is, the areas compare as follows, as regards: 
HIGHEST. LOWEST. 
INTENsITY, (d) White, (a) Yellow, (4) Blue, (c) Dark, 
i llow, 
ATTRACTIVENESS, (4) Blue, (æ) White, f a be 
1 The colors of the glasses used in the experiments, as tested by the spectroscope, were 
as follows : - 
RED.—Transmits red and a little orange. Complete absorption of the upper end down 
to a little beyond the Dline. Lower end just percéptibly shortened. 
YELLow ; i 
absorption of upper end down to å in the green. Red end very slightly shortened. Two 
layers cut out of the upper end as far as E, but still transmit some green. Three 
BLUE.—Transmits blue, indigo, and violet, and a very little green and red. Ina single 
int transmissio between E and 
green nearly. With three or four layers nothing is visible below F. 
