The Origin of Species: By Means of Natural Selection of the Preservation of Favoured Races in the Struggle for Life. –
-- Summary by Susan Fleck

Summary of Chapter 9 – Hybridism:

1. Considering all the ascertained facts on the intercrossing of plants and animals, it may be concluded that some degree of sterility, both in first crosses and in hybrids, is an extremely general result; but that it cannot, under our present state of knowledge, be considered as absolutely universal.

2. The sterility is of all degrees, and is often so slight that the most careful experimentalists have arrived at diametrically opposite conclusions.

3. There are cases, in which two pure species can be united easily and produce numerous hybrid offspring, yet these hybrids are sterile. But, there are species which can be crossed very rarely, or with extreme difficulty, but the hybrids, when at last produced, are very fertile. Even within the limits of the same genus, these two opposite cases occur.

a. The fertility, both of first crosses and of hybrids is more easily affected by unfavorable conditions, than is that of pure species.

4. The sterility is innately variable in individuals of the same species, and is eminently susceptible to the action of favorable and unfavorable conditions.

5. The degree of sterility does not strictly follow systematic affinity, but is governed by several complex laws.

6. The greater or less facility of one species to unite with another is incidental on unknown differences in their reproductive systems. There is no more reason to think that species have been specially endowed with various degrees of sterility to prevent their crossing and blending in nature, than to think that trees have been specially endowed with various and somewhat analogous degrees of difficulty in being grafted together in order to prevent their inarching in our forests. [inarch – graft (a plant) by connecting a growing branch without separating it from its parent stock]

7. [Several examples given in the chapter.] The foregoing rules and facts, on the other hand, appear to me clearly to indicate that the sterility both of first crosses and of hybrids is simply incidental or dependent on unknown differences in their reproductive systems. [genetics]