THE PURE LINE HYPOTHESIS. ^63 



To bring;' Blending- Inheritance into line with Mendelian 

 Irheritance, we have simply to suppose that the slightly different 

 factors, corresponding- to the two characters which can blend, 

 influence each other when combined by cross-fertilisation in the 

 same pair of homologous chromosomes, and become similar to 

 each other, and more or less intermediate in nature between the 

 factors of the two characters (see plate). 



5. According- to most Mendelians, the so-called fluctuating 

 or casual variations seen among individuals having the same 

 gametic nature are due to changes in the environment, and are 

 non-inheritable, since the factors or potentialities of the germ- 

 cells of the individuals are considered to be identical and prac- 

 tically unchangeable. With such a view, Johannsen's conception 

 of pure-lines in self-fertilised generations is a necessary conse- 

 quence. According to this, the selection of variations in a series 

 of self-fertilised generations can have no effect, since the factors 

 on which heredity depends are supposed to remain unchanged 

 from generation to generation. It is a question which can only 

 be decided by experimentation, and Karl Pearson ihas shown 

 that Johannsen's results are inconclusive. 



6. The question whether factors can be modified by the selec- 

 tion of individuals exhibiting small variations is of fundamental 

 importance, since if not, the origin of species and of adaptation 

 passes beyond our ken. The problem can only be solved by an 

 appeal to experiment, and it is presented in its simplest form by 

 utilising self-fertilised generations. Take a pure-bred plant, 

 self-fertilise it, and raise a family. A pure-bred plant is one 

 which, on self-fertilisation, or on mating with a like individual, 

 will only produce oft'spring that are all roughly similar to one 

 another without any tendency to break up into different types. 

 The individuals of the family will exhibit small variations which 



EXPLANATION OF PLATE. 



Plate 16 illustrates the hypothetical relationship between a character 

 and its factor in the body-cells and germ-cells in the case of both 

 Mendelian (or Xon-Blending) Inheritance and Blending Inheritance. 



On the left there is illustrated the crossing of a pure-bred purple fox- 

 glove with a pure-bred white plant. The grand-offspring resulting from 

 self-fertilisation are in the proportion of one deep-purple dominant (homo- 

 zygous) two intermediate or medium-purple (heterozygous), and one 

 pure white recessive (homozygous). The two different factors in the two 

 homologous chromosomes of the heterozygous offspring are supposed to 

 separate from each other without being mutually influenced. 



On the right there is shown the crossing of a dark-purple foxglo\re 

 with a pale-purple one, and the offspring are intermediate in colour. On 

 self- fertilising these, the next generation does not show any segregation 

 into plants exhibiting the characters of the grand-parents. It may be 

 supposed that the factors for dark-purple and light-purple influence each 

 other when combined in the same nucleus, and both become intermediate 

 and alike, and consequently in the subsequent separation of the members 

 of the pair of chromosomes containing the factors the germ-cells remain 

 all alike, and therefore the intermediate nature of the parent is handed 

 down to the offspring. 



