INHERITANCE ON CROSSING 57 



Each character is represented twice over, one of the 

 representatives having been obtained from the male 

 parent, the other from the female. The factors may 

 be in similar or in dissimilar pairs, according to whether 

 the male and female reproductive cells which formed 

 it were the same, or different. 



Finally, and perhaps most important of all, Mendelian 

 theory conceives the portion of germ plasm contained 

 in each reproductive cell, as having a perfectly definite 

 structure. If it is derived from a hybrid parent, it 

 may have either of two structures with regard to any 

 one factor, but it cannot be any vague or indefinite 

 mixture of the two. The value of this idea has been 

 compared to that of the atomic theory in chemistry. 

 It was this theory which, by giving chemists the idea 

 of definite chemical substances, made possible the 

 whole science of modern chemistry. There is a close 

 analogy between this and the principle of Mendehsm, 

 the principle of discontinuity and definiteness among 

 living organisms. 



As regards the real nature of what we have called 

 " factors " — ^the representatives, in the germ plasm of 

 the characters of the organism — it must be admitted 

 that we know nothing. Indeed, we know so httle of 

 the chemical nature of the Hving substance that it is 

 almost useless even to theorise on the subject. Much 

 laborious chemical research will have to be carried out 

 before the structure of the germ plasm, and its relations 

 to the mature organism, can be understood. 



We shall consider later the question of the general 

 appHcation of Mendel's law. In the meantime, we may 

 instance a number of characters which have been 

 showTi to be inherited according to the simple 

 MendeHan scheme. Such characters are colour in 

 mice, guinea-pigs, rabbits, fowls, and pigeons ; length 

 of hair in certain animals ; certain feather characters 

 in birds ; leaf form and leaf coloration in certain 

 plants ; flower colour in a great number of species ; 

 such important practical matters as the presence or 

 absence of horns in cattle, resistance to " rust " disease 



