Inheritance in the Higher Plants iii 



concrete terms. It is a theoretical, a pliilosophical, concep- 

 tion. If one crosses two individuals, the genes or gametic 

 factors common to each breed true. He can obtain an 

 idea of the behavior in heredity of those factors only which 

 are not common to the two parents. These factors segre- 

 gate and recombine in deiinite proportions. They follow 

 Mendel's law. The genotype conception of heredity is 

 therefore the conception that duplex or homozygous gametic 

 factors are due to produce identical results within the 

 limits of variability imposed by external conditions and 

 by the influence of other independent gametic factors during 

 ontogeny, no matter what is the appearance of the indi- 

 vidual from which they were derived. This is a strict 

 Mendelian conception of heredity extended to the organism 

 as a whole. 



We need not go into the many lines of work that support 

 the genotype theory. Considered in a broad way I believe 

 no reputable modern work is irreconcilable to it, although 

 some authors do not so interpret their work. All modern 

 plant breeding is in its support, for the principle of VUmorin, 

 the progeny test, which is the basis of all modern selection 

 work, is founded upon the same conception. In naturally 

 inbred plants, one has commercial strains which are mechani- 

 cal mixtures of near-homozygotes, and can be immediately 

 isolated. In naturally cross-bred plants or in bisexual ani- 

 mals, one has physiological mixtures, that is, hybrids or 

 heterozygotes, from which it takes somewhat longer to iso- 

 late particular strains that are genot}'picaUy homozygous in 

 respect to certain characters, but in which the separation 

 is accomplished by the same means^the breeding test. 



In the theoretical homozygous genotype transmissible 

 variations may occur, for no one believes protoplasm 



