CHAPTER IX. HYBRIDISM.
5. RECIPROCAL DIMORPHISM AND TRIMORPHISM. (continued)
We may, however, infer as probable from the consideration of dimorphic and
trimorphic plants, that the sterility of distinct species when crossed and
of their hybrid progeny, depends exclusively on the nature of their sexual
elements, and not on any difference in their structure or general
constitution. We are also led to this same conclusion by considering
reciprocal crosses, in which the male of one species cannot be united, or
can be united with great difficulty, with the female of a second species,
while the converse cross can be effected with perfect facility. That
excellent observer, Gartner, likewise concluded that species when crossed
are sterile owing to differences confined to their reproductive systems.