57 



A direct outcome of these investigations has been the establishing of 

 two new theories which may be regarded simply as further developments 

 or modifications of the Law of Mendel, as that law was first described. The 

 law itself is in no way altered by these developments; it is only shown to be 

 applicable to complex as well as to simple problems. The first of these The Theory 

 theories is known as the Theory of Presence and Absence and implies that f "Presence 

 the "Presence" of a certain unit or character with its corresponding "Absence " "^ ^ 

 together form, paradoxical though it may seem, a character pair. This idea 

 was first applied to plant life by Correns as a result of many years of most 

 exacting work, although Bateson, Punnett and E. R. Saunders were the first 

 to fully recognize the principle and to develop it as a new and consistent 

 theory. 



This theory will be better understood when we remember that Mendel 

 considered there to be in the gamete a definite something corresponding to the 

 dominant character or a definite something corresponding to the recessive 

 character. In no case, however, could these coexist in a single gamete. 

 For these somethings the term Factor has come to be commonly used. 



Mendel believed that the gamete always carried a definite factor cor- 

 responding to either the dominant character or the recessive character of a 

 character-pair. No gamete however, could carry more than one of the two 

 factors belonging to such a pair, by reason of which fact the characters were 

 said to be alternative to each other. This conception has undergone a slight 

 modification within recent years owing to the number of cases which it was 

 unable to explain. This difficulty was met in a simple way by the theory 

 of Presence and Absence. 



Some excellent illustrations of the manner in which this theory may be 

 applied, together with the difficulties which it seems to elucidate, are afforded 

 by Nilsson-Ehle in crossings between different sorts of oats and wheat. Thus 

 in crossings between certain black and yellow-grained oat sorts, white-kernell- 

 ed individuals appeared regularly in the second generation. According to ideas 

 which prevailed before exact experimental data were available, these white 

 grained forms would be regarded either as ' reversions' to the character of 

 a former parent, to the sudden reappearance of a previously latent character 

 or perhaps to something quite new. Not only did new forms arise in these 

 crossings but the proportions into which the hybrids grouped themselves 

 showed that the combination was not a simple monohybrid one. 



A concrete example is afforded in the crossing made at Svalof between 

 the yellow-grained oat sort No. 0875 and the Black sort No. 0401 (49 p. 44) . 

 In the second generation there were found in one case 155 Black grained 

 plants, 43 yellow-yellowish and 15 white or in the proportion of 10.3 black; 

 2.9 yellow-yellowish; 1 white. By grouping the yellow and white grained 

 forms together we have the proportions 2.7 black; 1 yellow-white. Of the 

 above 213 plants, 185 -were reasonably well developed. When the seed of 

 the latter came to be sown out in separate plots there were obtained the 

 following: 



