438 Evolution and Adaptation 



Castle assumes, in fact, that in animals like daphnids and 

 rotifers one polar body only is extruded, and the other (the 

 second) is retained in the egg, and hence the potentiality of 

 producing males is present. In the honey-bee, on the con- 

 trary, Castle assumes that both polar bodies are extruded 

 in the unfertilized egg (and there are some observations that 

 support this idea), and since only males are produced from 

 these, he believes it is the female element that has been sent 

 out into the second polar body. This hypothesis is necessary, 

 because Castle assumes that when both elements are pres- 

 ent in the bee's eggs, the female element dominates. " Hence, 

 if the egg which has formed two polar cells develops with- 

 out fertilization, it must develop into a male. But if such an 

 egg is fertilized, it invariably forms a parthenogenetic female 

 9 ( o* ), that is, an individual in which the male character is 

 recessive. Accordingly the functional spermatozoon must 

 in such cases invariably bear the female character, and this is 

 invariably dominant over the male character when the two 

 meet in fertilization." 



If it should prove generally true that the size of the egg 

 is one of the factors determining the sex, we have still the 

 further question to consider as to whether some eggs are 

 bigger because they are already female, or whether all eggs 

 that go beyond a certain size are females, and all those that 

 fail to reach this are males. If this is the case, an animal 

 might produce more females if the external conditions were 

 favorable to the growth of the eggs, and if in some cases 

 these large eggs were capable of developing, parthenogenetic 

 races might become established. Should, however, the condi- 

 tions for nutrition become less favorable, some of the eggs 

 might fall below the former size and produce males. It is 

 not apparent, however, why all the fertilized autumn eggs of 

 the aphids should give rise to females, for although these 

 eggs are known to be larger than the summer eggs, yet they 

 are produced under unfavorable conditions. 



