What is Darwin presenting in chapter 8?
In chapter 8, Darwin addresses the fourth problem of his theory – sterility of cross breeding of species. The main question he is trying to address is why is there generally sterility when crossing species. Natural selection cannot explain this since, by definition, sterility is not inheritable.
He states that there has been more success with crossing plants than animals in achieving fertility with different species.
“In regard to animals, much fewer experiments have been carefully tried than with plants.”
“I doubt whether any case of a perfectly fertile hybrid animal can be considered as thoroughly well authenticated.”
“I do not know of any thoroughly well-authenticated cases of perfectly fertile hybrid animals”
He gives some examples of animals that are “close” being crossed to produce fertile offspring.
“I have some reason to believe that the hybrids from Cervulus vaginalis and Reevesii, and from Phasianus colchicus with p. torquatus and with p. versicolor are perfectly fertile.”
And later he points out that plants are much easier to cross.
“It can be shown that plants most widely different in habit and general appearance, and having strongly marked differences in every part of the flower, even in the pollen, in the fruit, and in the cotyledons, can be crossed. Annual and perennial plants, deciduous and evergreen trees, plants inhabiting different stations and fitted for extremely different climates, can often be crossed with ease.”
But, he does not explain how significantly different animal species cannot cross to produce (fertile) offspring. And he does not even attempt to address this. But only considers animals that are similar.
So, it goes back to the first sentence in the chapter that remains the best explanation.
“The view generally entertained by naturalists is that species, when intercrossed, have been specially endowed with the quality of sterility, in order to prevent the confusion of all organic forms.”