148 DARWINISM AND HUMAN LIFE 



noting that the offspring of carefully pure-bred 

 types might be almost rephcas of the parents, 

 so we must notifce the opposite extreme, where 

 the offspring represent something quite new — a 

 novel position of organic equiUbrium— a " freak," 

 or " sport," or " mutation," or " discontinuous 

 variation." That these new departures have some- 

 times formed the beginning of a new domesticated 

 breed or cultivated variety is well known ; and 

 it is possible that species in nature may sometimes 

 have arisen in a similar way. 



(7) Mendelian Inheritance. — In typical cases 

 of Mendehan inheritance we have to do with the 

 pairing of two pure-bred types which differ from 

 one another in respect of one or more unit char- 

 acters, which may be obvious qualities, such as 

 colour and markings, or more subtle qualities, 

 such as the loaf-producing " strength " of wheat, 

 its susceptibility or immunity in respect to rust, 

 the broodiness or non-broodiness of poultry, 

 the horned or hornless state of the head in 

 cattle. 



The result of the crossing is that the " hybrid " 

 progeny all resemble one parent in respect of 

 the contrasted characters. There are no inter- 

 mediates, for Mendelian characters do not blend. 

 The offspring of grey and white mice are all grey ; 

 the offspring of giant and dwarf peas are all tall ; 

 and so on. It is usual to speak of the character 

 that persists and is expressed as the dominant 

 character, while that which remains imexpressed 

 or latent is called recessive. 



But when the " hybrids " are inbred, the next 

 generation shows a reappearance of the original 

 parental types both dominant and recessive — 



