550 



HEREDITY 



and feathers in chickens, etc. In man many characters, among 

 which are insanity and susceptibility to tuberculosis, are known 

 to behave according to Mendel's law, and Eugenics, which has 

 to do with applying the laws of heredity in a way to produce a 

 healthier and a more efficient race of men, has its chief sup- 

 port in Mendelism. 



Mendel's discoveries have already enabled us to make some 

 notable achievements in the way of improving plants and ani- 

 mals. By working according to the Mendelian conception, 

 many desirable varieties of the cereals, much more desirable 



FIG. 479. Height of plants in the F 2 generation of 'Tom Thumb Pop 

 Corn (a dwarf Corn) crossed with Missouri Dent (a large Corn). The plant 

 at the extreme right is similar in height to the dwarf parent, while the one at 

 the extreme left is similar in height to the Missouri Dent. After Emerson and 



East. 



ornamental plants, and various kinds of better fruits have been 

 developed. 



Of course in the extended investigations in Genetics since 

 1900, many situations have arisen that Mendel did not meet 

 Cases have arisen in which the Mendelian behavior can be 

 explained better by assuming that pairs of contrasting characters 

 are due to the presence and absence of certain factors and not to 

 dominant and recessive factors. According to the latter hypothe- 

 sis, the tallness of the tall variety of Peas is due to the presence 

 of a factor for tallness, while dwarfness in the dwarf variety is 



