^^ ZOOGENESIS "^"^^ 



In certain insects, as in the social ants, bees and 

 wasps, and in the termites or white-ants, individuals 

 occur — the so-called workers — which have non-func- 

 tional reproductive organs. In the ants, bees and 

 wasps such individuals are always females, but in 

 the termites they are both males and females. 



Corresponding non-breeding forms are not infre- 

 quent in other types of insects, and also in birds and 

 mammals, though in these so far as known they 

 serve no useful purpose. Unsexed polyps or cor- 

 responding units are of course common in the coelen- 

 terates and in the polyzoans, and occur also in some 

 tunicates. 



The production of fertile eggs or living young by 

 females only is so extensively developed in some types 

 of animals that sexual reproduction is a very rare 

 occurrence — indeed in some it is entirely unknown. 

 In others it is casual or rare. Between the two 

 extremes almost every type of intermediate exists. 



But throughout the animal world sexual reproduc- 

 tion is the usual and most widespread means of insur- 

 ing the perpetuation of the species, and even in those 

 animal types with extensive asexual reproduction or 

 reproduction through females only sexual reproduc- 

 tion almost without exception plays an important 

 part. 



Asexual reproduction or division may take place at 

 any stage, though it is most frequent in the early 

 stages and in young animals. In the case of reproduc- 

 tion through the development of unfertilized eggs the 

 eggs are usually produced by fully grown females, 



