314 APPENDIX 



to make it possible to determine the number of different forms 

 under which the offspring of hybrids appear, or to arrange these 

 forms with certainty according to their separate generations, or 

 definitely to ascertain their statistical relations.^ 



It requires indeed some courage to undertake a labour of such 

 far-reaching extent; this appears, however, to be the only right way 

 by which we can finally reach the solution of a question the impor- 

 tance of which cannot be overestimated in coimection with the 

 history of the evolution of organic forms. 



The paper now presented records the results of such a detailed 

 experiment. This experiment was practically confined to a small 

 plant group, and is now, after eight years' pursuit, concluded in all 

 essentials. Whether the plan upon which the separate experiments 

 were conducted and carried out was the best suited to attain the 

 desired end is left to the friendly decision of the reader. 



Selection of the Experimental Plants 



The value and utility of any experiment are determined by the 

 fitness of the material to the purpose for which it is used, and thus 

 in the case before us it cannot be immaterial what plants are 

 subjected to experiment and in what manner such experiments 

 are conducted. 



The selection of the plant group which shall serve for experiments 

 of this kind must be made with all possible care if it be desired to 

 avoid from the outset every risk of questionable results. 



The experimental plants must necessarily — 



1. Possess constant differentiating characters. 



2. The hybrids of such plants must, during the flowering period, 

 be protected from the influence of all foreign poUen, or be easily 

 capable of such protection. 



The hybrids and their offspring should suffer no marked disturb- 

 ance in their fertility in the successive generations. 



Accidental impregnation by foreign pollen, if it occurred during 

 the experiments and were not recognized, would lead to entirely 

 erroneous conclusions. Reduced fertility or entire sterility of cer- 

 tain forms, such as occurs in the offspring of many hybrids, would 

 render the experiments very difficult or entirely frustrate them. In 



^ [It is to the clear conception of these three primary necessities that the whole 

 success of Mendel's work is due. So far as I know this conception was absolutely 

 new in his day.] 



