386 V. THE NATURE OF EVOLUTION 



tion of an inviable zygote. Hybrid sterility prevents the reproduction 

 of hybrids that have reached the developmental stages at which the 

 parents normally breed. Sterile hybrids produce either no functional 

 gametes or gametes that give rise to inviable zygots. 



It should be noted, however, that while wild species, as a rule, 

 thus cannot be crossed, domestic species can usually be bred among 

 themselves. This indicates that wild species cannot exist as such 

 unless the crossing with the original species becomes impossible, 

 whereas domestic species can be preserved without the reduction in 

 fertility owing to the isolation by artificial selection. 



