BRliLDING SYSTEMS 



develop. They are sexual males. At the same time females arc 

 produced whose eggs undergo regular mciosis and are therefore 

 sexual. Both types undergo perfect reduction, but only the X-bearing 

 sperm of the males function. The haploid eggs after fertilization, 

 therefore, give only females, which re-establish the parthenogenetic 

 summer series (cf Fig. 65). 



Fig. 65. — The effect of temperature on sex determination in the egg of the psychid 

 moth Talaeporia tubidosa where the female with one X is heterogamctic according 

 to Sciler, 1920. At the higher temperature or when over-ripe the unpaired X 

 chromosome passes more often to the egg than to the polar body at the first meiotic 

 division and therefore gives a preponderance of males. An adaptable sex-ratio is 

 important in these moths where the female is immobile. 



This whole process of cyclical parthenogenesis is a means of 

 economizing on sexual reproduction during the summer period of 

 rapid expansion of numbers. If, of course, such a species moves 

 south into a region of perpetual summer, as seems to happen in 

 the United States, then the species will not enjoy its sexual season. 

 It had achieved facultative apomixis but it will have had obligatory 

 apomixis thrust upon it. 



Quite a different story is it with those plants and animals which 

 find themselves suddenly deprived of means of regular sexual 

 reproduction by the circumstances of their birth. A triploid of the 



264 



