可以类比.I take it for granted you have read this book.中it是形式宾语,真正的宾语从句就是：you have read this book(前面的引导词that省略了）.
在We take it for granted the two most common forms of transmission——by means of sounds and by visual signs.中,it要去掉,因为the two...signs不是个句子而是一个较长的词组作宾语,由于较长,避免头重脚轻,故放到了句末.一般形式宾语it多指代句子、动词不定式和动名词.故此处不能用it.