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 



28-VI 



30-VII 



15-IX 



Fig. 53. — Morphological cycle of head height in Hyalodaphnia. Roman 

 numerals designate months. {From Babcock and Clausen, after WoUereck.) 



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 



