172 MODES OF RESEARCH IN GENETICS 



respect of all other characters, it is "pure" and 

 "pure-bred" so far as concerns comb. Is it not 

 a contribution of moment to the breeder to have 

 demonstrated that in his breeding operations he 

 may safely and surely deal with individual char- 

 acters, and groups of correlated characters as units ? 



(c) That in a very great range of cases, per- 

 haps in all — the number of known cases daily 

 grows larger — the Mendelian law of segregation 

 and recombination of characters operates. In the 

 formation of the germ cells of an individual there 

 is a sorting out or segregation of the hereditary 

 characteristics contributed by the father and the 

 mother and a readjustment of these into all of the 

 combinations, both old and new, which are mathe- 

 matically possible. What may be the precise cel- 

 lular mechanism or basis of this wonderful process 

 is not altogether certain, but the phenomenon it- 

 self is as certain as the phenomenon of gravitation. 

 It operates as well in regard to the minutest heri- 

 table differences in the pedigreed specimens of the 

 same sub-breed as in the wide differences of true 

 hybridization. Properly understood, it enables 

 the breeder to interpret and weigh the results of 

 his breeding operations, and so intelligently to 

 plan the next steps with a certainty and precision 

 hitherto unattainable. Is not this a real contri- 

 bution of science to practice ? 



(d) That the germinal bases of heritable unit 

 characters can be changed or altered in any respect, 



