Appendix C. 175 



which are, on the other hand, so much inclined to hybri- 

 dization, afford a good example of this theory, which appears 

 to be confirmed also by Lobelia, Passiflora, and Oncidium " 

 {American Naturalist, vol. viii, pp. 223-4, 1874). 



Different groups of plants exhibit remarkable differences in 

 the capability of their constituent species to hybridize. — In so 

 far as these differences have reference only to first crosses, 

 they have no bearing either for or against my theory. Only 

 in so far as the differences extend to the production of fertile 

 hybrids does any question arise for me. First of all, therefore, 

 I must ascertain whether (or how far) there is any correlation 

 between groups whose species manifest aptitude to form first 

 crosses, and groups where first crosses manifest aptitude 

 to produce fertile hybrids. Next, whatever the result of this 

 inquiry should be, if I find that certain natural groups of 

 plants exhibit comparatively well-marked tendencies to form 

 fertile hybrids, the question will arise. Are these tendencies 

 correlated with paucity of species? If they are, the fact 

 would make strongly in favour of physiological selection. 

 For the fact would mean that in these natural groups, owing 

 to " the nature of the organisms " included under them, less 

 opportunity is given to physiological selection in its work of 

 differentiating specific types than is given by other natural 

 groups where the nature of the organism renders them more 

 prone to mutual sterility. But in prosecuting this branch 

 of verification, I must remember to allow for possibilities of 

 differential degrees of geographical isolation in the different 

 groups compared. 



On this subject Focke writes me as follows: — "In a 

 natural group (family, order, genus) showing considerable 

 variability in the structure of the flower, we may expect 

 to find [or do find] a greater number of mules than in 

 a group whose species are only distinguished by differ- 

 ences in the shape of the leaves, or in growth, &c. I do not 



