DIMINISHING ENVIRONMENTAL INFLUENCE 321 



15 to 20 days, by subjection to a temperature of 25° to 30°. On the other 

 hand, some eggs from the same original pair of A. fasciata, which, though 

 exposed to the same high temperature, developed more slowly — the larval period 

 taking 142 to 163 days, and the pupal 25 to 31 days — yielding specimens having 

 a wing measurement of 55 to 57 mm. 17 



It will be seen that these variations are not so great as those among 

 plants or among snails. 



There is, nevertheless, one very striking and curious kind of per- 

 manent modification possible among insects. I refer to the artificial 

 production of royal couples among ants, and also to the production of 

 queens among bees, both brought about by differences in the food 

 supply. It would appear that the factor which determines whether a 

 young female bee becomes a queen or a worker is whether she is, or 

 is not, fed upon the royal food. In the case of ants this is not so clear, 

 for the results may depend largely upon original differences in the 

 eggs. Even if these transformations are brought about solely by en- 

 vironmental alterations they need not have much bearing on the present 

 discussion, since this unusual possibility is of adaptive value to the 

 species and may have been especially evolved and maintained. This 

 is utilized by the insects themselves, " If an old queen dies, or if by 

 accident all the prospective queens have been lost." 18 



Experiments which bring about changes in the life-cycle, and also 

 those which produce parthenogenesis in insects which are normally 

 sexual, can not be considered surprising. As to changes in the life- 

 cycle, the case of the rose aphid is a good illustration. This insect pro- 

 duces parthenogenetically female offspring during the summer months, 

 and normally on the appearance of winter begins to produce both males 

 and females. This winter condition may be indefinitely postponed if 

 the animals are kept in a suitable environment with plenty of food and 

 water. Here we are not witnessing a profound modification, but only 

 the failure of a certain normal change to take place in the failure of 

 its normal stimulus. 



Quite separate and different from this is the question of producing 

 segmentation and growth in an unfertilized egg by some artificial chem- 

 ical or physical means. This is called artificial parthenogenesis, and to 

 have any special bearing on the present discussion must be produced 

 in eggs that normally require fertilization by spermatozoa. The silk- 

 worm happened to be one of the first of the invertebrates to lend itself 

 to this form of modification. " In 1886 Tichomiroff published the 

 fact that the unfertilized eggs of the silkworm Bombyx mori, can 

 be caused to develop by rubbing them gently with a brush, or by 

 putting them for a short time into concentrated sulphuric acid." 19 



17 Vernon, " Variations," p. 230. 



"See Morgan, "Exp. Zool.," pp. 317-320; and Vernon, p. 287. 



19 Loeb, " Dynamics," p. 165. 



