INTERNAL CAUSES OF VARIATION 179 



asexually reproduced is not ''immensely reduced below the vari- 

 ability of the race." 1 



In the honeybee only the male sex is produced parthenoge- 

 netically. In plant lice it is commonly the female alone under 

 high temperature, and both sexes under lower. Weismann bred 

 separately two varieties of Cypris reptans for some seven years, 

 covering more than forty generations and " many thousand indi- 

 viduals." One variety, A, was light in color; the other, B, was 

 dark. No males were ever discovered in either, and it is sup- 

 posed that this species produces only parthenogenetically. While, 

 for the most part, the descendants of each were extremely alike, 

 yet " minute differences invariably existed." 



Not only was this true, but in 1887, three years after the 

 experiment commenced, " some individuals of the dark green 

 variety, B, appeared in the aquarium with the light variety." 

 The same variation appeared a second and a third time, and in 

 the last instance intermediate forms could be made out. In 

 1891 another case occurred, and in the same year its converse 

 appeared, a few typical light individuals among the dark 

 colony that had "bred true many years." 2 Does this experi- 

 ment also throw light on the origin of varieties, and were these 

 mutations ? However this may be, it clearly shows that vari- 

 ability is not entirely dependent upon sexual union, and that 

 even distinct varieties may arise without the intervention of sex. 



The first significant fact in maturation of parthenogenetic 

 eggs is that they produce but one polar body. 3 



From this point on two alternatives seem possible. In the 

 first place, a second polar body appears to be forming in the 

 usual manner and the separation of the nuclear matter takes 

 place, but instead of passing out of the egg it remains behind, 

 fusing again with the nucleus of the egg proper, which straight- 

 way undergoes development, with its chromosomes increased to 



1 Pearson, Grammar of Science, pp. 472-473. 



2 Weismann, Essays on Heredity, I, 161-164. 



3 Often this polar body divides, giving the appearance of two, but a second one 

 is not formed. It is quite remarkable, though entirely consistent, in the case of 

 aphis, honeybees, and certain other forms that produce both sexually and asexu- 

 ally, that \\\Q fertilized eggs produce two polar bodies but the parthenogenetic eggs 

 only one. 



