Forbes also thought he saw, as he later told the British Association, that "the number of species and individuals diminishes as we descend, pointing to a zero in the distribution of animal life as yet unvisited."
您好,曹老师,我想请教一下这个句子里的as yet unvisited中的as是什么词性,表示什么意思?如何理解本句中as+过去分词这种结构?