species Hybrids. — Goodspeed and Clausen of the Cal . 

 Exp. St. have formed a definite hypothesis for the interpre- 

 tation of such phenomena as the "mutation" behavior of 

 Oenothera, bud variation in horticultural plants, and the 

 complex relations exhibited in species hybridization. The 

 main points of the hypothesis may be outlined as follows : 



a. The hereditary units of species form complex but 

 harmonious reaction systems, the specific elements of which 

 are the factors of Mendelian heredity. 



b. Strict Mendelian inheritance follows hybridization 

 between forms which have fundamentally like reaction - 

 systems. The Mendelian behavior is a consequence of the 

 existence of specific differences in certain elements in the 

 common reaction-system. 



c. When fundamentally different, but equivalent, re- 

 action systems-are contrasted, as in some species hybrids, 

 the elements of the two systems may not be interchangeable 

 as in Mendelian hybrids because the diverse elements may 

 form inharmonious reaction-systems. These inharmon- 

 ious systems either may not function at all or may function 

 in an abnormal fashion. The results of this difficulty are 

 expressed in a variety of ways, particularly in the marked 

 sterility of species hybrids and in the production of ab- 

 normal individuals. 



4. — Linkage and Crossing-over 



It will be observed after a study of the preceding abnormal 

 cases of crossing that Mendelian views have changed with 

 the progress of investigations, especially after the discovery 

 that both the factors and the chromosomes undergo segre- 

 gation and show parallelism in their methods of distribu- 

 tion. Determiners have taken the place of factors or char- 

 acters, each trait being represented in the germ plasm by its 

 own determiner. 



With the more extended investigations, it became ne- 

 cessary to evolve the factorial hypothesis to explain the re- 

 sults. According to this hypothesis each visible character 

 is due to the action of a large number of factors or genes in 

 the germ plasm, each factor in turn influencing a large num- 

 ber of other traits. Moreover, these factors or genes are 

 linked together in groups, in chromosomes where they are 

 arranged in a linear series, but sometimes changing places 

 by "crossing-over". Besides each factor or gene has a 

 definite location in its chromosome, in a chromomere. As 

 each chromosome carries several factors it is to be expected 



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