316 APPENDIX 



those individuals belong to a species which under precisely the 

 same circumstances display precisely similar characters, no two of 

 these varieties could be referred 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 ques- 

 tion. 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 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 generations. The experiment 

 resolves itself therefore into just as many separate experiments as 

 there are constantly differentiating characters presented in the 

 experimental plants. 



The various forms of Peas selected for crossing showed differences 

 in the length and colour of the stem; in the size and form of the 

 leaves; in the position, colour, and size of the flowers; in the length 

 of the flower stalk; in the colour, form, and size of the pods ; in the 

 form and size of the seeds; and in the colour of the seed-coats and 

 of the albumen [cotyledons]. Some of the characters noted do not 

 permit of a sharp and certain separation, since the difference is of a 

 " more or less " nature, which is often difficult to define. Such 

 characters could not be utilised for the separate experiments ; these 

 could only be applied to characters which stand out clearly and 

 definitely in the plants. Lastly, the result must show whether they, 

 in their entirety, observe a regular behaviour in their hybrid unions, 

 and whether from these facts any conclusion can be come to re- 

 garding those characters which possess a subordinate significance 

 in the type. 



