58 Heredity. 



may be produced by isolated females whose mothers and 



/randmothers had been isolated all their lives. 

 Glaus has shown that the eggs begin to develop in 

 the female Evadne, a form closely related to Daphnia, 

 before the animal is born ; and impregnation would here 

 seem to be impossible. 



In Daphnia and related forms the parthenogenetic 

 eggs appear to give rise to females only, but as the males 

 are very rare indeed, as compared with the females, it 

 is difficult to show that they never originate by parthe- 

 nogenesis, for the evidence is only negative. Schaffer, 

 the discoverer of parthenogenesis in Daphnia, also dis- 

 covered that Apus, a crustacean which belongs to an- 

 other order, lays eggs which give rise without impreg- 

 nation to fertile females, and that this may go on for 

 an indefinite number of generations. In Apus, and in 

 most of its allies, the males are extremely rare, al- 

 though the females may be very abundant, and one ob- 

 server, Joly, found only one male specimen of Artemia 

 salina among 3000 females. 



j Parthenogenesis is known to occur in many insects. 

 It is rare and exceptional in some of them, while in 

 others it is as frequent and normal as it is in Daphnia. 



Among the butterflies and moths, sexual union is the 

 rule, and parthenogenesis a rare exception, but in 1701 

 Dr. Albrecht made the remarkable discovery that a female 

 Bombyx, which had escaped from its pupa under a 

 glass shade, and which could not have been visited by 

 a male, laid fertile eggs. As sexual union is known to 

 be almost universal in the Bombycidae, this observation 

 was at first discredited, but the phenomenon has in 

 more modern times been observed with every possible 

 precaution in Bombyx mori by a number of most com- 

 petent observers, among whom are Schmidt, Earth el- 



