CAUSE OF HYBRID VIGOR 173 



that characters are inherited in groups. The different 

 theories accounting for this linkage of factors make no 

 essential difference in the use to which these facts will be 

 put here. It is only necessary to accept as an established 

 fact that characters are thus inherited and that it is these 

 groups of factors which Mendelize. The chromosome 

 view of heredity will be used, as it is the most probable, 

 the -most useful, and permits representation in the sim- 

 plest manner; but adherence to this view is not necessary 

 for our purpose. 



The increasing complexity of Mendelism points very 

 strongly to the probability that the important characters 

 of an organism are determined, or at least affected, by 

 factors represented in practically all of the chromosomes 

 or linkage groups. This is comprehensible when it is re- 

 membered that height or any other size differentiation 

 is only an expression of an organism's power to develop. 

 Hereditary factors which affect any part of the organism 

 may indirectly determine the maximum of any size char- 

 acter. For example, in plants height is governed by root 

 development as well as by that of the aerial parts. 



The widespread occurrence of abnormalities and char- 

 acters which are detrimental to an organism's best devel- 

 opment are well known. It may be taken for granted, 

 nevertheless, that no one individual has all the unfavor- 

 able characters, nor, on the other hand, all the favorable 

 characters known to occur in the species. For the most 

 part, each possesses a random sample of the good and the 

 bad. This being true, it is only necessary to assume that 

 in general the favorable characters are in some degree 

 dominant over the unfavorable, and the normal over the 



