Fallible. The chapter on "miscellaneous objections to the theory of natural selection" (chapter VII in my edition) may have been dropped from your edition or added by the time mine came out. I certainly can't think of any reason to include such a chapter these days!

His chapter on hybridism deals with the idea that hybrids between varieties or species covers the whole spectrum from increased fertility to total infertility. It's a good argument against those who believe species are fixed entities. I'll get onto it ASAP and get back to you.