为什么是generally speaking而非general ,speaking究竟是动名词还是现在分词?副词可以修饰分词吗?

后者.
如：
geographically speaking
从地理的角度来说
例句
The publishing house reportedly retorted that geographically speaking,north China is correct.这家出版社则巨力反驳,【从地理的角度来说】,华北也是正确的.
Geographically speaking 【从地理上说】,the spread of bird flu appears to be following the migratory routes of wild birds
同理,generallyadv.通常；普遍地,一般地
generally speaking一般来说；一般说来；一般而言；一般地说