PLANT HYBRIDIZATION BEFORE MENDEL 291 



"in order to discover the relations in which the hybrid forms stand 

 toward each other, and also toward their progenitors, it appears to be 

 necessary that all members of the series developed in each successive 

 generation should be, without exception, subjected to observation." (th.^ 

 P- 337.) 



Mendel's attention was called to the Leguminosae as a possible 

 group for experimentation, because "of th^ir peculiar floral struc- 

 ture." (p. 337.) After making experiments with several members 

 of this family, he came to the conclusion that the genus Pisum 

 (pea) fulfilled his requirements. He investigated, during two 

 years, thirty-four more or less distinct varieties of peas obtained 

 from seedsmen. Twenty-two of these varieties "were selected and 

 cultivated during the whole period of the experiments. They re- 

 mained constant without an exception.'* (p. 338.) 



Mendel concerned himself little with the supposed systematic 

 classification of his varieties of peas. The majority of them he 

 assigns to Pisum sativum, others to sub-species of this, and still 

 others to distinct species. 



"The positions, however, which may be assigned for them in a classi- 

 ficatory system are quite immaterial for the purpose of the experiments 

 in question. It has so far been found to be just as impossible to draw a 

 sharp line between the hybrids of species and varieties, as between 

 species and varieties themselves." (p. 338.) 



The earlier hybridizers of plants, for the most part, made a 

 distinction between "hybrids" so-called, between "species," and 

 "crosses" between "varieties." Mendel discards this terminology, 

 recognizing that the distinction is one of degree and not of kind, 

 a distinction essentially artificial when closely applied. 



The fundamental difference between Mendel's hybridization 

 experiments and all others stands out most clearly in the follow- 

 ing statement : 



"if two plants which differ constantly in one or several characters be 

 crossed, numerous experiments have demonstrated that the common 

 characters are transmitted unchanged to the hybrids and their progeny; 

 but each pair of differentiating characters, on the other hand, unite in 

 the hybrid to form a new character, which in the progeny of the hybrid 

 is usually variable. The object of the experiment was to observe these 

 variations in the case of each pair of differentiating characters, and to 

 deduce the law according to which they appear in the successive genera- 

 tions. The experiment resolves itself therefore into just as many sepa- 

 rate experiments as there are constantly differentiating characters pre- 

 sented in the experimental plants." (pp. 338-9.) 



