338 Mendel's Experiments 



In all, thirty-four more or less distinct varieties of Peas 

 were obtained from several seedsmen and subjected to a 

 two years' trial. In the case of one variety there were 

 noticed, among a larger number of plants all alike, a few 

 forms which were markedly different. These, however, 

 did not vary in the following year, and agreed entirely 

 with another variety obtained from the same seedsman ; 

 the seeds were therefore doubtless merely accidentally 

 mixed. All the other varieties yielded perfectly constant 

 and similar offspring ; at any rate, no essential difference 

 was observed during two trial years. For fertilisation 

 twenty-two of these were selected and cultivated during 

 the whole 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 



