PRINCIPLES OF HEREDITY 217 



In many ways Chapter IT. is an'important one, since it describes 

 the nature of the material with which Mendehan experiments 

 have dealt, and to which Mendelian principles have been shown to 

 apply. Some of the characters, the inheritance of which have 

 been studied by Mendelian methods, are not only structural in 

 nature, but several of them are physiological. The difierent 

 habits assumed by plants are among some of the characters 

 which have been investigated. Some individuals in a race are 

 tall and others dwarf ; some have a branching habit, others are 

 unbranched ; some straggle and others are erect ; some are 

 biennial and others annual ; some wheats are susceptible to the 

 attacks of the " rust-fungus," while others are immune. These 

 habits, with the exception of the biennial and annual habits, 

 and the straggling and erect habits, which require further investi- 

 gation, behave in inheritance according to Mendelian principles. 

 Among animals, the peculiar round-about motion of waltzing 

 mice and the normal habit are known to be transmitted in 

 Mendelian fashion. Hairiness and smoothness of the epidermis 

 of plants, the prickliness and smoothness of fruits, lax and 

 dense ears in wheat, starchy endosperm and sugary endosperm, 

 short hair and long hair in rabbits and guinea-pigs, the various 

 forms of combs in fowls, and eye-colour and various abnormalities 

 in man, and a large number of other characters, for which the 

 reader should refer to the book itself, have already been shown 

 to behave in inheritance as Mendelian factors. 



How extensive has been the work which has dealt with the 

 transmission of colour in plants, animals., and Man alike, may be 

 gathered from the fact that four chapters, namely, four to eight 

 inclusive, are AvhoUy concerned in describing the phenomena 

 relating to colour-transmission. Perhaps in no other character is 

 a knowledge of inheritance so certain, and of such a great degree 

 of refinement, as in that of colour. Heredity and sex, and some 

 interesting problems related to the subject, are dealt with in 

 Chapter X. The existence of Mendehan inheritance in Man is 

 discussed in Chapter XII. In Chapter XV. the bearing of 

 Mendehan principles upon old biological conceptions is dealt with 

 in a tentative and suggestive manner. Here we find a discussion 

 of the nature of Mendelian units and of segregation ; the possible 

 moment at which segregation occurs is indicated ; the nature of 

 reversion and variation, discontinuity in variation and the 

 relationship of Mendelism to Natural Selection is dealt with in 

 a lucid and inspiring fashion. 



The final or sixteenth Chapter deals with the practical appli- 

 cation of Mendelian principles. In this chapter we are introduced 

 to several extremely interesting matters. We learn that our old 

 conception of a pure-bred race or individual must be profoundly 



