THE "PRESENCE AXD ABSENCE" 

 HYPOTHESIS 1 



DR. GEORGE HARRISON SHULL 

 Station for Experimental Evolution, Carnegie Institution op 

 Washington 



In Mendel's 2 discussion of the behavior of character- 

 istics in the offspring of splitting hybrids, the phenomena 

 of segregation are described in terms of pairs of antago- 

 nistic characters. He assumed that these are represented 

 by pairs of internal units, one member of each such pair 

 coming from one parent, the other from the other parent. 

 This idea of pairs of characters in Mendelian hybrids has 

 been generally entertained until somewhat recently, and 

 is still perhaps not uncommonly held. De Vries 3 made 

 use of this conception in stating what he thought to be a 

 fundamental distinction between species and varieties, 

 assuming that the differentiating features of varieties 

 are represented by units which are homologous with corre- 

 sponding units of the species from which such varieties 

 sprang, and which are paired with those units on crossing, 

 while different species lack such homology and pairing 

 of determiners. 



About six years ago in a paper on Mirabilis crosses, 

 Correns 4 stated the members of several Mendelian "pairs 

 of characters," as the presence and absence of single 

 characters. Cuenot 5 in a paper doubtless written simul- 



1 Read before the Botanical Society of America, at Baltimore, December 

 31, 1908. 



2 Mendel, J. G. "Versuche iiber Pflanzen-Hybriden. ' ' Verhandlung des 

 Naturforscher-Yereines. Briinn IV. 47 pp., 1866. 



3 De Vries, H. "Species and Varieties: Their Origin by Mutation," 

 pp. 847, 1904. See p. 251 et seq. 



4 Correns, C. "Weitere Beitrage zur Kenntnis der dominierenden 

 Merkmale und der Mosaikbildung der Bastarde." Bcr. d. deutsch. Bot. 

 Ges. 21: 195-201, Ap. 23, 1903. 



5 Cuenot, L. "L'heredite de la pigmentation ehez les souris" (2me 



