49 



ceeding generations. Others segregate or divide producing constant (ho- 

 mozygous) and inconstant (heterozygous) combinations. The latter continue 

 to segregate until so reduced in proportion to the constant forms as to finally 

 become practically lost sight of in the case of normal self -fertilizers. Thus 

 in the end are to be found a whole host of constant combinations each of 

 which, further crossing excluded, breeds true in succeeding generations. 



The constancy of pure lines in self fertilizing species of plants seems to 

 displace at one stroke practically all previous conceptions regarding the 

 question of variation. We must abandon the idea that all life is in a con- 

 stant state of unrest, always varying this way or that. " Had this analytical 

 principle " says Johannsen, " been used in the times of Darwin or had it even 

 been appreciated by the Biometrician school certainly the real bearing of 

 selection might long since have been rightly understood" (18 p. 143). 



By reason of the variability of soil, moisture, light and other external Individual 

 factors there are always to be found a larger or smaller number of individuals and partial 

 within a pure line which deviate from the common type. There are also to modifications 



be found variations between certain parts of individuals. The first form of m pure 



. . . lines. 



variation can best be designated as individual variation (modification), and 



the latter as partial modification. Neither, however, is regarded as hereditary. 

 These modifications may be sufficient to cause certain individuals, within 

 a given strain to " transgress " or " over-lap " those in another. Thus a plant 

 belonging to a certain strain may become so altered by external conditions 

 as to become apparently identical with that belonging to another. An 

 excellent example is afforded in connection with the various strains taken 

 out of the Probstier oats. The length of each individual in six of these 

 strains is indicated in the following diagram (42, p. 128) ; 



A study of the above diagram clearly indicates that a direct botanical 

 examination of a common population can give scarcely more than an indi- 

 cation of its constitutents. Only by the separate culture of a sufficient 

 number of individuals and by a determination of the average condition of 

 each character in the progeny can an effective analysis be made. It is this 

 average condition which distinguishes one strain from all others. This fact 

 constitutes a second great reason why the isolation of superior mother plants 



