346 APPENDIX 



With regard to the form of the hybrids and their development, 

 as a rule an agreement with the observations made in Pisum is 

 unmistakable. It is otherwise with the exceptional cases cited. 

 Gartner confesses even that the exact determination whether a 

 form bears a greater resemblance to one or to the other of the two 

 original species often involved great difficulty, so much depending 

 upon the subjective point of view of the observer. Another cir- 

 cumstance could, however, contribute to render the results fluctu- 

 ating and uncertain, despite the most careful observation and 

 differentiation. For the experiments, plants were mostly used 

 which rank as good species and are differentiated by a large number 

 of characters. In addition to the sharply defined characters, where 

 it is a question of greater or less similarity, those characters must 

 also be taken into account which are often difficult to define in 

 w^ords, but yet suffice, as every plant specialist knows, to give the 

 forms a peculiar appearance. If it be accepted that the develop- 

 ment of hybrids follows the law which is valid for Pisum, the series 

 in each separate experiment must contain very many forms, since 

 the number of the terms, as is known, increases, w^ith the number 

 of the differentiating characters, as the powers of three. With a 

 relatively small number of experimental plants the result therefore 

 could only be approximately right, and in single cases might 

 fluctuate considerably. If, for instance, the two original stocks 

 differ in seven characters, and 100 or 200 plants were raised from 

 the seeds of their hybrids to determine the grade of relationship of 

 the offspring, we can easily see how uncertain the decision must 

 become, since for seven differentiating characters the combination 

 series contain 16,384 individuals under 2187 various forms; now 

 one and then another relationship could assert its predominance, 

 just according as chance presented this or that form to the observer 

 in a majority of cases. 



If, furthermore, there appear among the differentiating char- 

 acters at the same time dominant characters, which are transmitted 

 entire or nearly unchanged to the hybrids, then in the terms of the 

 developmental series that one of the two original parents which 

 possesses the majority of dominant characters must always be pre- 

 dominant. In the experiment described relative to Pisum, in 

 which three kinds of differentiating characters were concerned, all 

 the dominant characters belonged to the seed parent. Although 

 the terms of the series in their internal composition approach both 



