The two most popular explanations of origin are that the belief goes back to pagan times when we believed in tree spirits, or that we are invoking Christ’s protection by referring to the wood of the Cross. The former is nothing but guesswork, based on the conviction that all superstitions must be ancient, and it has the usual problem of spanning thousands of years with no evidence at all of its existence, or, for that matter, any evidence that ‘we’ ever believed in tree spirits.
Source: Computational Materials Science, Volume 267
，这一点在爱思助手下载最新版本中也有详细论述
在山西运城市平陆三湾大天鹅景区栖息的白天鹅。
cat access.log | grep "error" | sort | uniq -c