452 THE MAINTENANCE OF SPECIES 



is a case of complementary factors, because one factor is required to 

 complement the other in order to bring the character into expression 

 while neither is effective alone. 



Different Kinds of Factors. There are also supplementary 

 factors, where one factor alone may produce a visible effect, but a 

 second factor may change its manifestation ; or inhibiting factors, 

 where the expression of a factor is prevented by the interference of 

 another; or duplicate factors, where separate "doses" of the same 

 thing combine to produce a cumulative effect ; or lethal factors, which 

 are so disharmonious that if they arrive together from both parental 

 sources, the unfortunate individual sooner or later dies, although able 

 to survive when only a single lethal factor comes from one parent ; 

 or sex-linked factors, that are tied up with either the maternal or the 

 paternal side of the house. In all these cases the factors in their 

 behavior obey the fundamental Mendelian laws, although the resulting 

 ratios furnish intriguing complications that Mendel himself did not 

 anticipate. 



It is hoped that the reader will be stimulated to explore in books 

 devoted primarily to Genetics (see bibliography) further than the 

 general survey presented in this chapter. 



Practical Breeding 



Selection 



Long before Mendel pointed the way by which to control the 

 operations of heredity, man was active in fixing desirable characters in 

 animals and plants by means of artificial selection, and in doing this 

 was only following in the footsteps of Mother Nature, who has been 

 exercising ''natural selection" from time immemorial. Many of the 

 forms selected and nurtured by man never could have survived if left 

 to the more exacting demands of nature. 



We know today, thanks to Mendel, that phenotypes do not always 

 reproduce their own kind, and that the genotype is the all-important 

 thing to get at in heredity. It must be admitted, however, that in 

 spite of difficulties encountered, our pre-Mendelian forebears, in estab- 

 lishing lines of domesticated animals and cultivated plants by the 

 method of blind selection of phenotypes, attained a remarkable degree 

 of success. Even the ancient lake-dwellers of prehistoric Switzerland, 

 it is said, developed ten different kinds of cereals from wild plants. 



There are three different methods of phenotypic selection which are 



