24 THE FIRST PRINCIPLES OF HEREDITY 



viduals, being male and female at the same time, are 

 known as " Hermaphrodites." 



Hermaphroditism is, as already mentioned above, a very 

 common occurrence in flowering plants, most flowers con- 

 taining pistil as well as stamen. 



In animals it is less frequent, but by no means rare. It 

 occurs normally in Sponges, Corals, Worms, Snails, etc. 

 Even the higher animals, including man, pass through a 

 foetal stage when both sex-organs, male and female, are 

 still existent ; but only one of them in the course of de- 

 velopment reaches its full form and function. As a patho- 

 logical curiosity, this embryonic stage may persist in the 

 adult. 



We find that even when both kinds of germ-ceUs are 

 present in the same individual self-fertilization is by no 

 means the rule. On the contrary, as Darwin has shown in 

 plants — and something similar holds good for animals — 

 most ingenious contrivances are set up to insure cross- 

 fertilization — i.e., fertilization of one individual by another. 



(c) Parthenogenesis. 



Parthenogenesis, which was discovered in 1745 by Bonnet 

 in the plant-lice, is the power which certain female animals 

 possess of producing offspring without sexual union with 

 the male — i.e., without fertilization. 



The case of the bee is a well-known example of^partial 

 Parthenogenesis. The queen-bee, who is impregnated only 

 once in her life — during her nuptial flight — lays two kinds 

 of eggs : one kind, which are fertilized by the queen-bee 

 with the stored-up sperm, become workers or a queen, 

 while the other kind, not fertilized, become drones. 

 Seasonal Parthenogenesis is to be found among water-fleas 

 (minute aquatic Crusters — Cladocera) and the plant-lice, 

 or Aphides. Here we have a succession of virgin-births 

 during summer, but males generally reappear towards 

 autumn, and with them the ordinary sexual reproduction. 



♦^fiOPfci 



**Acoa^j 



V-Of 



f'^iBRAfiy, 



