As the great scroll of history unrolls, many complicated incidents occur which it is difficult to introduce effectively into the pattern of the likes and dislikes of the epoch in which we live.
▲分析：这个 it 并非多余，所以不可省略——因为这里的 which 并非主语（其后的 it 才是定语从句的主语，或叫形式主语，真正的主语是后面的不定式 to introduce...），它其实是动词 introduce 的宾语，简单地说，原句在句法上可以作如下理解：
which it is difficult to introduce effectively into the pattern of the likes and dislikes of the epoch in which we live （这是一个which 引导的定语从句，修饰名词...incidents）
→it is difficult to introduce which effectively into the pattern of the likes and dislikes of the epoch in which we live（这里的 which 是指前面提到的...incidents）
▲句子大意：当历史巨大的卷轴展开之时，人们会看到许多错综复杂的事件，而这些事件很难有效地进入我们所处这个时代人们的好恶模式之中。
▲类似的例句如（请注意体会 which 在定语从句充当什么句子成分）：
(1) Washington is unhappy with the peace plan which it views as unworkable. 美国政府对和平计划不满，认为它不可行。
(2) It is required by law to shut down banks which it regards as chronically short of capital. 法律规定资金长期短缺的银行必须关闭。
(3) The council has some spare cash which it proposes to spend on public amenities. 政务委员会有一些余款，计划用来增添一些公共便利设施。