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 

 dominants yield dominants and recessives in the 

 old proportions of 3 : 1. 



It is supposed that the hybrids have germ-cells 

 of two kinds, one half 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 unlike combination 

 occurring that is, of normal and waltzing as of 

 the like 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. 



OCCURRENCE OF MENDELIAN INHERITANCE. 

 Mendelian 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, peculiar 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 



