PLANT HYBRIDIZATION BEFORE MENDEL 303 



"whoever studies the coloration which results in ornamental plants 

 from similar fertilization can hardly escape the conviction that here 

 also the development follows a definite law which possibly finds its ex- 

 pression in the combination of several independent color characters." 

 (P- 370.) 



This leads to a reference to the matter of what are com- 

 monly known as "unit characters." Whatever these unit character- 

 determinants or genes may be, they are probably of the nature of 

 factors, the release of the operation of which sets in train a series 

 of physiological changes, which ultimately wind up by producing 

 the visible structural characters in question, and which are seen 

 to function as units in the cross. This conception was, as a matter 

 of fact, Mendel's own. The fact, furthermore, that in the produc- 

 tion of many complex characters several factorial units may 

 share, Mendel himself also surmised. 



Mendel's conclusion, then, from his peas hybrids is as fol- 

 lows ( 1 ) : 



"It is now clear that the hybrids form seeds having one or other of 

 the two differentiating characters, and of these one-half develop again 

 the hybrid form, while the other half yield plants which remain constant, 

 and receive the dominant or the recessive characters (respectively) in 

 equal numbers." (p. 349.) 



Since the offspring of hybrids split off or segregate to the extent 

 of one-half in each succeeding generation, an example of the result 

 in respect to the seeds is given by Mendel as follows: 



Dr r Ratios 



2 



4 



8 

 16 

 32 



"in the tenth generation, for instance, 2'' — 1 = 1023. There results, there- 

 fore, in each 2,048 plants which are in-, this generation, 1,023 with the 

 constant dominant character, 1,023 with the recessive character, and only 

 two hybrids." (p. 350.) 



Mendel thus demonstrated that the hybrid character originally 

 brought together by crossing cannot be "fixed" as a whole by 

 selection. Each succeeding generation of the close-fertilized prog- 

 eny will undergo a constant diminution of the number of the 



