140 PLANT HYBRIDIZATION BEFORE MENDEL 



continued selection, succeeds in impressing upon a "variety" the 

 stamp of a certain type and, through repeated and continuous 

 selection in the same direction, finally "fixes" it, so that the 

 variety becomes, as it were, stabilized. 



It probably usually means that, by continuous selection of 

 some certain type, those individuals are usually isolated, which 

 are more or less homozygous for the character-units thus repre- 

 sented, and which become "fixed" because no heterozygous fac- 

 tors are left to split apart. 



We have here, in other words, an unscientific expression, 

 through practical experience, of the fact which the breeder of 

 today would define as the selection of a heterozygote having 

 dominant characters differing from those of the species. Being 

 of hybrid nature, such a plant would break up, and hence yield 

 to selection, whereas the plants resembling the type, being more 

 apt to be homozygous, would be less liable to vary in their prog- 

 eny. He emphasizes the view just set forth still more emphati- 

 cally in the following words : 



"if two variations are produced, of which the one differs little from 

 the type, but is placed upon the line which leads in the desired direction, 

 and the other is placed in an opposite direction, but departing consider- 

 ably from the type, we shall not neglect nevertheless to follow this 

 latter, because with it the breaking-up of atavism is more advanced." 

 (6, p. 31-) 



The necessity of fixing upon some single individual plant, as the 

 basis of selection, is referred to by Verlot in the following terms : 



"We ought then to recognize that it is necessary to take account for 

 the choice of the seed-bearers, not only of the external characters, but 

 even of the idiosyncrasy of each one of them. Now, since this does not 

 manifest itself except by its effects, we shall, if a variation seems to 

 present some difficulties in becoming fixed, have to examine separately 

 the products of each of the seed parents, and make our choice bear upon 

 those which present, in the least pronounced degree, atavism or the 

 tendency to return to the primitive type." (6, p. 32.) 



Verlot's experience with and observations upon hybrid plants, 

 as coming from an experienced horticulturist, are valuable, espe- 

 cially to the practical plant breeder. 



Regarding the now well-understood fact of the gradual disap- 

 pearance of the hybrid form through segregation, he says : 



"Their fertility is of short duration, through the more or less rapid 

 return of their products to the types which have given them birth." 

 (6, p. 25.) 



