154 DARWINISM AND HUMAN LIFE 



normal mice all the progeny are normal. The 

 waltzing habit is recessive, the normal is dominant. 

 When the hybrids are inbred the resulting genera- 

 tion consists of normal mice and waltzing mice in 

 the proportion of three dominants to one recessive. 

 The . recessives of this generation, when inbred, 

 yield only recessives, for as many generations as 

 one likes to breed them. The dominants are 

 found to be of two kinds : one-third of them — 

 called pure dominants — when inbred yield only 

 dominants ; the other two-thirds — called impure 

 domirmnts — ^yield dominants and recessives in the 

 old proportions of 3 : 1. 



It is supposed that the hybrids have germ-cells 

 of two kinds, one haK bearing the waltzing charac- 

 ter, the other the normal character. Each germ-cell 

 is " pure " as regards this character. There are 

 twice as many chances of the unhke combination 

 occurring — that is, of normal and waltzing — as of 

 the hke combination occurring — that is, of normal 

 meeting normal, or waltzing waltzing. In other 

 words, the percentage of individuals in the three 

 groups will be what it is: 25 pure normal, 50 

 impure normal, and 25 pure waltzing. 



OCCURBENCB OF MeNDELIAN INHERITANCE. — 



MendeUan phenomena are known in rats, mice, 

 rabbits, guinea-pigs, poultry, canaries, snails, silk- 

 worms, and some other animals ; in peas, beans, 

 stocks, wheat, barley, maize, and some other 

 plants. The characters which illustrate it are 

 such as size, colour, markings, crests, horns, hairi- 

 ness, pecuhar features such as the waltzing habit 

 in mice, and elusive properties, such as broodiness 

 in hens, time of ripening and immunity in wheat. 

 It is doubtful how far Mendelian phenomena 



