NATURAL SELECTION 397 



Sterility of Hybrids. Such sterility and interspecific sterility 

 are probably due to many causes. Now that the minute structure 

 of the germ cells is known, we realize that the mechanism of 

 reproduction differs in various species as well as gross morphology. 

 Even in similar species this mechanism may differ, and the associa- 

 tion and separation of chromosomes may take place, if it takes 

 place at all, in such an irregular way that the normal process 

 of reproduction is prevented. The mere fact that species have 

 arisen from other species does not involve the maintenance of 

 fertility between the divergent ))ranches; if they differ in one way 

 they may well differ in others. 



Neuter Castes. In social insects, such as the ants and ])ees, 

 neuter individuals do not ordinarily reproduce, but they are 

 merely non-functional females and may in certain cases lay eggs. 

 Worker bees do so when the colony becomes queenless and there 

 is no way to rear a new queen. In such cases the eggs laid by the 

 workers are not fertilized, since the insects never mate, and can 

 develop only into drones. There is possibility here for the trans- 

 mission occasionally of the specializations of the so-called neuter 

 caste, but it is doubtful that the conditions would occur often in a 

 natural state. Even discarding this possibility, however, we find 

 in more primitive species of the same order an explanation of the 

 development of worker structures and instincts. The functional 

 female, or queen bumblebee, not only acts as the mother of the 

 colony, but also carries on all of the duties of workers until she 

 has reared a colony to relieve her of such work. It is highly 

 probable therefore that the characters of workers appeared before 

 limitation of reproduction to one or a few individuals. This 

 would imply that no further change could occur in the workers, 

 but we must recognize that the queens vary, and that there is 

 possibility in this variation for the selection of characters to be 

 expressed in the workers. On the basis of such selection carried 

 on by bee-keepers, various strains have been established, as noted 

 in Chapter XIX. The difference between this and non-social 

 species is that the colony becomes the biological unit, instead of 

 the pair. Colonies are preserved or destroyed through the qualities 

 of the workers, which in turn are derived from the queen. The 

 individual is wholly subordinated to the welfare of the group. 



Vestigial Organs. Probal)ly the most important objection to 

 natural selection is its failure to explain the persistence of vestigial 



