Chromatin as ''Hereditary Substance'' 355 



actually proved connection between hereditary attributes 

 and clironiosomes. Morgan's hypothesis is, however, so in- 

 teresting and seems so likely to be proved partly true (that 

 is, true to the extent of there l)eing some sort of connection 

 between the attributes in (Question and chromosomes), that 

 it seems desirable to present the most salient parts of the 

 theory. 



That the case is, as above indicated, still in the hypotheti- 

 cal stage, seems not to be appreciated by some enthusiasts, 

 though fortunately Morgan is not one o£ these. In the 

 preface to the volume The Mechanism of Mendelian Heredity 

 jNIorgan writes : "But it should not pass unnoticed that 

 even if the chromosome theory is denied, there is no result 

 dealt with in the following pages that may not be treated 

 independently of the chromosomes ; for we have made no 

 assumption concerning heredity that cannot also be made 

 abstractly without the chromosomes as bearers of the postu- 

 lated hereditary factors." ^^ 



The observations on which Morgan's hypothesis rests be- 

 long to two very different categories, and these categories 

 pertain to parts of the organism anatomically far away 

 from each other, namely, to the union and separation or 

 "segregation" of the hereditary attributes of adult organ- 

 isms that propagate bisexually, and to the union and sep- 

 aration of chromosomes of the germ-cells during maturation 

 and fertilization in these same organisms. As an illustration 

 of the first part of this statement take one of Mendel's own 

 cases, that of the table pea having roundish seeds crossed 

 with a variety having angular and deeply wrinkled seeds. 

 All the seeds of plants arising immediately from this cross 

 (the Fj generation) are round. But if now the plants of 

 this Fi lot are pollinated among themselves, their immediate 

 progeny (the F2 generation) will have both round and 

 angular seeds in the proportion of three round to one angu- 

 lar. In a word, since the F2 plants produce seeds corre- 



