He was appointed to the position by virtue of his extensive experience in the field.

by 是介词，在这里表示"通过...方式"或"凭借"。 virtue 本义是"美德、优点"，引申为"优势、长处"。 of 是介词，表示所属关系或来源。 by virtue of 是一个固定短语，意思是"凭借...的优势"或"由于...的原因"，强调某种特质或条件带来的结果。