14 A MANUAL OF MENDELISM 



(1) Pure species produce constant or true-breeding 

 forms, and must therefore be furnished with the materials 

 for doing so. 



(2) Hybrids also produce pure-breeding forms, and 

 also must be furnished with the materials for doing so. 



(3) Hybrids produce true-breeding forms in all the 

 combinations in which the characters carried by their 

 parents can be combined, and therefore must be 

 furnished with the materials to produce all these true- 

 breeding forms. 



(4) Hybrids produce equal numbers of these true- 

 breeding forms, and therefore must be furnished with 

 equal numbers of the materials for the production of 

 their characters. 



Since the characters distributed among the true- 

 breeding forms are the same as those in which their 

 grandparents differed, Mendel's hypothesis might be 

 rendered : Hybrids must be furnished with equal 

 numbers of the materials for the production of the 

 differentiating characters carried {a) by their true-breed- 

 ing progeny or (b) by their parents. 



As yet, however, this last statement is no more than 

 a theory, a hypothesis, which must be tested and 

 proved before being raised to a higher level. Mendel 

 remarked that it might be proved " theoretically," that 

 is without further experiment, but did not stop to do 

 so ; and, before proceeding to his experimental proof, it 

 will be well to consider how this may be done. 



Consider first the case in which the parents differ in 

 one pair of characters : in one having round seed while 

 the other has wrinkled. The hypothesis is that, in 

 their hybrids, the ovaries, on the one hand, and the 

 anthers, on the other, contain equal numbers of the 



