208 OUTLINES OF EVOLUTIONARY BIOLOGY 



substitute not a for b without altering the result, and the differences 

 between the paternal and maternal chromosomes of any given pair 

 may depend merely upon the absence from one or the other of 

 one or more factors which are present in its mate. 



Mendelian phenomena are often greatly complicated and 

 rendered very difficult of interpretation by the fact that the 

 activity of a given factor, or its power to influence the development 

 of the zygote, may be dependent upon the presence of another 

 factor belonging to a different " allelomorphic pair." In other 

 words, a given character may depend not merely upon the 

 presence of a single factor in the zygote but upon the co-operation 

 of two or more factors, and if these factors happen to be separated 

 in the process of reduction and do not happen to come 

 together again in the union of the gametes to form the zygote, 

 then the character in question * will not appear in the organism 

 developed from the zygote. In this way it may happen that a 

 character which was present in the ancestors of a particular 

 organism may disappear for many generations and then suddenly 

 re-appear as the result of some cross or hybridization in which 

 the necessary factors happen to be brought together again in the 

 zygote. In this way are explained those cases of " reversion " to 

 ancestral types which Darwin found to occur so frequently as the 

 result of cross-breeding. 



One of the most thoroughly investigated cases of this kind 2 is 

 that of certain white sweet peas belonging to the variety known 

 to horticulturists as "Emily Henderson." Plants of this variety 

 are not really all alike but differ from one another in the shape of 

 their pollen grains, and when the two kinds, distinguishable only 

 in this way, are crossed, the offspring are frequently found to 

 possess purple flowers resembling those of the wild Sicilian pea 

 from which our cultivated varieties have been derived. It is not 

 necessary to go into the somewhat complicated analysis of this 

 case in any detail, but it has been demonstrated by the researches 

 of Professor Bateson and Professor Punnett that the re-appearance 

 of the lost colour in the hybrid is due to the re-union in the zygote 

 of certain factors which had become separated at some period or 

 other in the ancestral history of the white-flowered parents. 



The existence of interactions of this kind between the different 

 factors or determinants in the germ plasm, giving rise often to 



1 Professor Bateson terms such characters " compound characters." 



2 Bateson, op. cit., pp. 89 et seg. 



