No. 637] FIBST GENEKATION HYBRIDS 1 25 



tion similar to that of the older writers who stated that 

 inbreeding was the ''cause" of degeneration. There 

 may well be other and important ways in which inbreed- 

 ing is associated with degeneration. 



There is for example definite evidence that vigor is 

 reduced by continued asexual reproduction (Shull, 1912), 

 and although it may be urged that this result is asso- 

 ciated with the phenomenon of senescence, so may be the 

 decline of vigor that follows inbreeding. 



Furthermore, it has been sho^\^l by Calkins (1919) 

 that in the ciliate, Ursoleptus, conjugation between sis- 

 ter cells of an asexually propagated line increases vigor. 



Objections to the Explanation 



Two objections have stood in the way of accepting 

 dominance as an explanation of heterosis. The first of 

 these is that if this explanation is the correct one it 

 should be possible to obtain an occasional F2 individual, 

 homozygous for all dominant allelomorphs, the progeny 

 of which should be uniformly as vigorous as the Fj. It 

 is held that no such Fo individual has been found. The 

 second objection is that the distribution in Fo should be 

 skew with the mode above the mean while in fact Fj 

 populations show a symmetrical distribution. 



Jones has proposed a novel and ingenious explanation 

 of both objections. He has pointed out that it is only 

 necessary to assume that the phenomenon of linkage, 

 which plays such an important role in the inheritance of 

 Drosophila, is operative also in maize. 



If groups of characters are inherited as units with 

 little or no crossing over, botih dominant and recessive 

 characters being represented in any particular unit, the 

 first generation would still exhibit all the dominant char- 

 acters of both parents, but when segregated into pure 

 lines each pure line would again exhibit recessive char- 

 acters, with a consequent decline in vigor. 



This assumption of group inheritance or linkage would 

 also meet the objection, that F.. populations exhibit a 

 normal and not a skew distribution. 



