APPENDIX 315 



order to discover the relations in which the hybrid forms stand 

 towards each other and also towards 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. 



At the very outset special attention was devoted to the Legu- 

 minosae on account of their peculiar floral structure. Exjjeriments 

 which were made with several members of this family led to the 

 result that the genus Pisum was found to possess the necessary 

 qualifications. 



Some thoroughly distinct forms of this genus possess characters 

 which are constant, and easily and certainly recognizable, and 

 when their hybrids are mutually crossed they yield perfectly fertile 

 progeny. Furthermore, a disturbance through foreign pollen 

 cannot easily occur, since the fertilising organs are closely packed 

 inside the keel and the anther bursts within the bud, so that the 

 stigma becomes covered with pollen even before the flower opens. 

 This circumstance is of especial importance. As additional advan- 

 tages worth mentioning, there may be cited the easy culture of these 

 plants in the open ground and in pots, and also their relatively short 

 period of growth. Artificial fertilisation is certainly a somewhat 

 elaborate process, but nearly always succeeds. For this purpose 

 the bud is opened before it is perfectly developed, the keel is 

 removed, and each stamen carefully extracted by means of forceps, 

 after which the stigma can at once be dusted over with the foreign 

 pollen. 



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 num- 

 ber of plants all alike, a few forms which were markedly dift'erent. 

 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 oftspring; 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 



