Lecture X 

 Mendel's law of balanced crosses 



In the scientific study of the result of crosses, 

 the most essential point is the distinction of the 

 several characters of the parents in their combi- 

 nation in the hybrids and their offspring. 

 From a theoretical point of view it would be 

 best to choose parents which would differ only 

 in a single point. The behavior of the differen- 

 tiating character might then easily be seen. 



Unfortunately, such simple cases do not read- 

 ily occur. Most species, and even many ele- 

 mentary species are distinguished by more than 

 one quality. Varieties deviating only in one 

 unit-character from the species, are more com- 

 mon. But a closer inspection often reveals 

 some secondary characters which may be over- 

 looked in comparative or descriptive studies, 

 but which reassume their importance in experi- 

 mental crossings. 



In a former lecture we have dealt with the 

 qualities which must be considered as being due 

 to the acquisition of new characters. If we 



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