420 Evolution and Adaptation 



In the honey-bee the remarkable fact has been well estab- 

 lished that fertilized eggs give rise only to females (queens 

 and workers), while unfertilized eggs develop into males. 

 Whether a fertilized egg becomes a queen or a worker 

 (sterile female) depends solely on the kind of food that is 

 given to the young larva, and this is determined, in a sense, 

 entirely by the bees themselves. 



In plants also there are many cases of parthenogenesis 

 known. Some species of Chara when kept under certain 

 conditions produce only female organs, and seem to produce 

 new plants parthenogenetically. In this case it appears 

 that the same conditions that caused the plants to produce 

 only female organs may also lead to the development 

 of the egg-cells without fertilization. In fact it is only by 

 a combination of this kind that parthenogenesis could arise. 

 The result is similar when the eggs of insects produce only 

 females whose eggs are capable of parthenogenetic develop- 

 ment. If a case should arise in which only females appeared 

 whose eggs did not possess the power of parthenogenetic 

 development, the species would die out. 



In the green alga, Spirogyra, it has been found that if 

 conjugation of two cells is prevented, a single cell may be- 

 come a parthenogenetic cell. In a number of parasitic fungi 

 the male organs appear to be degenerate, and from the 

 female organs parthenogenetic development takes place. A 

 small number of flowering plants are also capable of par- 

 thenogenetic reproduction. 



There is a peculiarity in the development of the partheno- 

 genetic eggs of animals that will be more fully discussed later, 

 but may be mentioned here. Ordinarily an egg that becomes 

 fertilized gives off two polar bodies, but in a number of cases 

 in which parthenogenetic development occurs it has been found 

 that only one polar body is given off. It is supposed that in 

 such cases one polar body is retained, and that it plays the 

 same part as the entrance of the spermatozoon of the male. 



