VARIATION 315 
is possible on the one hand by an increase of temperature and of 
inorganic salts, and on the other hand by a decrease of carbon 
assimilation. 
b) Temperature and pigmentation. Many experiments in the 
rearing of moths and butterflies under controlled temperatures prove 
that degree of pigmentation is profoundly influenced by the tempera- 
ture at which the pupae are kept. Some species exhibit seasonal 
dimorphism in the wild state. By taking pupae of the common 
European form of the swallowtail butterfly, Papilio machaon, and 
subjecting them to a temperature of 37° to 38° C., Standfuss obtained 
the characteristic summer form which occurs in Palestine. Again it 
has been shown by temperature experiments that many variations 
3-VI 31 
18X 
28-VI 
30-VII 15x 
Fic. 53.—Morphological cycle of head height in Hyalodaphnia. Roman 
numerals designate months. (From Babcock and Clausen, after Woltereck.) 
found among insects in nature are merely aberrations due to tempera- 
ture effects. Goldschmidt by artificially controlled temperatures has 
produced a series of forms of the diurnal peacock butterfly, Vanessa io, 
which show the fading out of the “peacock eye” mark (see Fig. 52). 
c) Food and structure. Woltereck was able to prove that the form 
(hence the structure) of the fresh water crustacean, Hyalodaphnia, 
varies directly with the food supply. These minute animals produce 
many generations during a season and the successive generations from 
the same water exhibit a morphological cycle, the earlier and later 
generations having shorter heads and the generations produced from 
midsummer to autumn having longer ones. Fig. 53 is a reproduction 
of Woltereck’s diagram of the morphological cycle in Hyalodaphnia 
showing variation in head and shell length as found on successive 
