CHAPTER Xm 



THEORY TO ACCOUNT FOR THE RESULTS WHICH FOLLOW 

 A CROSS INVOLVING TWO PAIRS OF CHARACTERS 



The theory enunciated in Chapter XI., to account 



for the fact of segregation and the ratio in which the 



segregated characters occur has more an indirect 



practical value than a direct one. Its value in 



practice is the lesson which it teaches rather than 



the immediate practical efiects which it is capable of 



achieving. This lesson is that any attempt to predict 



the result of a given mating must be based on a 



knowledge of the character- factors contained in 



the germ cells of the individuals mated, and not 



merely on the visible characters of these individuals 



themselves. The scientific control of breeding must 



depend on a knowledge of characters as represented 



in the germ cells ; no amount of familiarity with 



the mere external features of the animals or plants 



dealt with will assist towards this control, except, 



of course, in so far as these external features afford 



a clue as to the contents of the germ cells within. 



The chief value, then, of the theory enunciated in 



the last chapter lies in the support which it affords 



to those new principles of breeding which look for 



guidance to the germ, and reject the indications of 



the body or soma, and are based partly on Mendel's 



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