In this fourth edition of his standard work on Genetics, William Castle has given a more extended account of human heredity, of which our knowledge has considerably increased in recent years. The chapter on the unit-characters of rodents has been completely rewritten, as what we know of heredity in mammals depends very largely on this group. Polyploidy, parthenogenesis, and the artificial production of mutations are also topics requiring new discussion. The views of biologists have reached substantial unity on the formerly controversial subject of selection, so that the treatment of this topic has been simplified.

For a study of the evolution of breeds, the rabbit is passed, since the parent species still exists wild and hybridization has not occurred with any second species to complicate the process. This discussion of the rabbit will therefore help the rabbit breeder better to understand the genetic constitution of his animals and so to be able to alter that constitution to suit his purposes. At the same time, students of genetics will find the information useful in enabling them to understand what rabbit breeders of this and previous centuries have accomplished and how they have been able to accomplish it.