Many creationists and other believers, who vehemently oppose the theory of evolution, have this strange, twisted concept that Charles Darwin invented this crazy theory and now the whole scientific community just believes it without question, like it was some kind of holy scripture. Therefore if you discredit Darwin or his writings, you discredit the entirety of evolution.
These people do not seem to realize nor acknowledge the humongous amount of research and testing that has been performed since Darwin, and how much the theory has actually evolved (no pun intended) since his publications. (For example, Darwin had no idea about DNA and how it can be used to create a cladistic tree, or trace back the ancestry of a species and connect it to other species. This being just one tiny example out of many.)
It seems to be that many of these people cannot get rid of a "prophet" mentality. In other words, that truth (or alleged truth) is handed down to ordinary people by wise "prophets", and this information is either the whole, unchanging truth, or completely false and devious. How to know which it is happens by examining the prophet himself. If the prophet is questionable in some way, or if anything that he said can be proven to be false, then everything that he said is dubious and should not be trusted. After all, the Bible itself talks about prophets and how to distinguish true and false prophets.
Curiously, many of these believers don't even themselves realize that they are engaging in this kind of thinking. Not even if it's explained to them. No matter how you explain to them that science just doesn't work that way, it doesn't help.