44 Mendel's Experiments 



period of the experiments. They remained constant without 

 any exception. 



Their systematic classification is difficult and uncertain. 

 If we adopt the strictest definition of a species, according 

 to which only those individuals belong to a species which 

 under precisely the same circumstances display precisely 

 similar characters, no two of these varieties could be re- 

 ferred to one species. According to the opinion of experts, 

 however, the majority belong to the species Pisum sativum ; 

 while the rest are regarded and classed, some as sub-species 

 of P. sativum, and some as independent species, such as 

 P. quadratum, P. saccharatum, and P. umbellatum. The 

 positions, however, which may be assigned to them in a 

 classificatory system are quite immaterial for the purposes 

 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. 



DIVISION AND ARRANGEMENT OF THE EXPERIMENTS. 



If two plants which differ constantly in one or several 

 characters be crossed, numerous experiments have demon- 

 strated that the common characters are transmitted un- 

 changed 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 generations. The 

 experiment resolves itself therefore into just as many 



