EXPERIMEXTS IN PLANT HYBRIDISATION. 



27 



but they possess greater variability of form and a more pronounced 

 tendency to revert to the original type. 



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 circumstance could, however, contribute to render the 

 results fluctuating 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 words, but yet suffice, as every plant connoisseur 

 knows, to give the forms a strange appearance. If it be accepted that the 

 development of hybrids follows the law which is valid for Pisum, the 

 series in each separate trial must embrace very many forms, since the 

 number of the components, as is known, increases wdth the number of the 

 differentiating characters in cubic ratio. With a relatively small number of 

 trial-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 and 200 plants were 

 raised from the seeds of their hybrids to determine the grade of relation- 

 ship of the offspring, we can easily see how^ uncertain the decision must 

 become, since for seven differentiating characters the developmental series 

 contains 16,384 individuals under 2,187 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 

 instances. 



If, furthermore, there appear among the differentiating characters at the 

 same time dominant characters, which are transferred entire or nearly 

 unchanged to the hybrids, then in the components of the developmental 

 series that one of the two original stocks which possesses the majority 

 of dominant characters must always be predominant. 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, i^lthough the components of the series in their internal compo- 

 sition approach both original stock plants equally, in this trial the type of 

 the seed parent obtained so great a preponderance that out of each 

 sixty-four plants of the first generation fifty-four exactly resembled it, or 

 only differed in one character. It is seen how rash it may be under 

 such circumstances to draw from the external resemblances of hybrid^i 

 conclusions as to their internal relations. 



Gartner mentions that in those cases where the development was 

 regular among the oft'spring of the hybrids the two original species were 

 not reproduced, but only a few closely approximating individuals. With 

 very extended developmental series it could not in fact be otherwise. For 

 seven differentiating characters, for instance, among more than 10,000 in- 

 dividuals — offspring of the hybrids — each of the two original species would 



