74 



In the case of cross-fertilizing species, where there is a constant re- 

 grouping of the units producing a continuous fluctuation in quantitative 

 characters, a distinction is also made between: 



(a) Fluctuating modifications. 



(b) Fluctuating variations. 



A final distinction is then made between plus and minus fluctuations 

 thus: 



(a) Plus and minus modifications, 



(b) Plus and minus variations. 



AN EXPLANATION OF THE OCCURRENCE OF CERTAIN ABERRANT FORMS 

 UNDER DOMESTICATION 



Lone wheat. 



dominant 

 heterozygotes 

 in field 

 cultures. 



In the light of the above investigations and conclusions an explanation 

 of the occurrence of certain strikingly aberrant types presents itself. Let 

 us consider first the well known case of the Cone or "Kotte" wheat (0319) 

 which sort was formerly thought to be a " sport " or mutation. The origin 

 of this sort is substantially as follows: In a pure line (0516), which was 

 characterized by long open heads and weak straw there suddenly appeared 

 in 1896, an aberrant form having an exceedingly close Cone-like head and a 

 stiff er straw than the former sort. The head density of the sort 0516 is given 

 in the Record Book as 21, while that of the aberrant type is recorded as 25. 

 In the records for 1897, the following annotations were made regarding the 

 progeny of the latter form : 



" Great variation, Red White; variations in type of head from Type IV to 

 Type I." In 1898, the following observations were recorded: "80% of the 

 heads with Brown chaff, 20% with White chaff; variations in type of head still 

 prevail.' 1 While the data available are still insufficient to permit a safe con- 

 clusion as to whether or not this form, which received the name Cone from 

 the cone-like type of head, is a mutation, the evidence seems to be against 

 such a conception since the progeny showed ordinary segregation of several 

 different characters with the production of different combinations in the same 

 manner as do artificial crossings. 



In the velvet-chaffed autumn wheat sort Pudel, although this is a pedi- 

 gree sort, there regularly appear smooth chaffed forms, often in considerable 

 quantities. An explanation of the appearance of these forms is found in 

 the ^ ac ^ that the sort in question has probably become accidentally cross- 

 fertilized by the pollen of smooth chaffed sorts growing in the Experimental 

 grounds. The fact that velvet-chaffed sorts are enormously in the minority 

 and are surrounded on all sides by sorts which are smooth-chaffed lends sup- 

 port to this opinion. This is still further strengthened by the following 

 fact 



