72 



constant (heterozygous) and consequently segregate in the next generation. 

 The remaining 25% resemble the characteristic false wild oat type (Fig. 13, c.). 

 These are constant, reproducing true to type in succeeding generations. 



From an economic standpoint False Wild Oats are not regarded in Scandi- 

 navia as being so objectionable as the true species. They are not found to 

 increase materially as they lack those qualities of the true species which 

 enable the latter to compete so successfully with the cultivated sorts. For this 

 reason this plant is ranked as a foreign variety rather than as a weed. 



From the above discussion it seems clear that aberrant forms may have 

 their origin not only through natural crossing but also as a result of certain 

 spontaneous changes in sorts which may ordinarily be regarded as constant. 

 Up to the present all changes of this nature recorded at Svalof are believed 

 to be of a Retrogressive kind. In no case has a character arisen which was not 

 formerly in existence, and in practical breeding work, it would seem that 

 none need be looked for. 



THE NATURE AND CLASSES OF VARIATION (SUMMARY). 



The most important conclusions as to the nature and classes of variation, 

 arrived at by Nilsson-Ehle (52, p. 3-19) as a result of his investigations with 

 wheat and oats, are summarized by him substantially as follows: 



1. In classifying the different kinds of variation the only division which 

 can be made is between variations induced by external factors (modifica- 

 tions') and hereditary variations. Both classes, modifications and hereditary 

 variations, may be either continuous or discontinuous. Hereditaiy variations 

 can be quite as "quantitatively continuous" as can modifications and in a 

 certain sense quite as fluctuating. 



2. The existence of quantitatively continuous, fluctuating, hereditary 

 variation as a ' combinations- variation ' has been proven through Mendelian 

 analysis in the same manner as has qualitative or discontinuous hereditary 

 variation. Thus one and the same external character such as red 

 color, may be due to the existence of several independent Mendelian factors 

 or units. It is characteristic for such factors to have practically the same, 

 or at most, a slightly different external effect and in combination with each 

 other actually to produce a cumulative effect. In this way the characters 

 concerned may become intensified. These factors may also be in a position 

 to alter the external effect of other factors. 



Through different combinations of different units a continuous line of 

 hereditary gradations (continuous hereditary variations) in respect of a given 

 character, is brought into existence. In due time the homozygous combina- 

 tions preponderate thus producing a line of constant forms, lines or biotypes. 

 These biotypes are therefore not the result of as many ' mutations ' but rather 

 of different combinations of already existing units. In the case of plants 

 which readily cross-fertilize, an inconstant (heterozygotic) condition is always 

 to be expected. In such cases the number of hereditary gradations from a 

 given number of units will be still more numerous and as a consequence here- 

 ditary variations will appear still more 'continuous/ 



