MENDEL 



sible care if it be desired to avoid from 

 the outset every risk of questionable 

 results. 



The experimental plants must neces- 

 sarily— 



1. Possess constant differentiating 

 characters. 



2. The hybrids of such plants must, 

 during the flowering period, be pro- 

 tected from the influence of all foreign 

 pollen, or be easily capable of such 

 protection. 



The hybrids and their offspring 

 should suffer no marked disturbance 

 in their fertility in the successive gen- 

 erations. 



Accidental impregnation by foreign 

 pollen, if it occurred during the experi- 

 ments and were not recognized, would 

 lead to entirely erroneous conclusions. 

 Reduced fertility or entire sterility of 

 certain forms, such as occurs in the 

 offspring of many hybrids, would 

 render the experiments very difficult 

 or entirely frustrate them. In 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 mem- 

 bers of the series developed in each 

 successive generation should be, with- 

 out exception, subjected to observa- 

 tion. 



At the ver\' outset special attention 

 was devoted to the Legimiinosae on 

 account of their peculiar floral struc- 

 ture. Experiments 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 rec- 

 ognizable, and when their hybrids are 

 mutually crossed they yield perfectly 

 fertile progeny. Furthermore, a dis- 

 turbance through foreign pollen can- 

 not easily occur, since the fertilising 



3 



organs are closely packed inside the 

 keel and the anther bursts within the 

 bud, so that the stigma becomes cov- 

 ered with pollen even before the 

 flower opens. This circumstance is of 

 especial importance. As additional ad- 

 vantages 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 cer- 

 tainly a somewhat elaborate process, 

 but nearly always succeeds. For this 

 purpose the bud is opened before it is 

 perfectly developed, the keel is re- 

 moved, and each stamen carefully ex- 

 tracted 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 dis- 

 tinct 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 there- 

 fore doubtless merely accidentally 

 mixed. All the other varieties yielded 

 perfectly constant and similar off- 

 spring; at any rate, no essential differ- 

 ence was observed during two trial 

 years. For fertilisation twenty-two of 

 these were selected and cultivated 

 during the whole period of the experi- 

 ments. They remained constant with- 

 out any exception. 



Their systematic classification is dif- 

 ficult and uncertain. If we adopt the 

 strictest definition of a species, accord- 

 ing to which only those individuals 

 belong to a species which under pre- 

 ciselv^ the same circumstances display 

 precisely similar characters, no two of 

 these varieties could be referred to one 

 species. According to the opinion of 



