How to understand the grammar in this sentence?
Here are the passages: "For more, check out Morgan’s work in full on the Internet Archive, which includes lovely passages like this one, marveling at how beavers build dams even though they don’t have to:

"As the dam is not an absolute necessity to the beaver for the maintenance of his life, his normal habitation being rather natural ponds and rivers, and burrows in their banks, it is, in itself considered, a remarkable fact that he should have voluntarily transferred himself, by means of dams and ponds of his own construction, from a natural to an artificial mode of life."

In the sentence: "it is, in itself considered, a remarkable fact that he should have voluntarily transferred himself,...", why did the author use "should have voluntarily" rather than "should voluntarily"? I mean "should have transferred" means the beaver didn't construct the dam, right? It's weird to me. "it is, in itself condiered", why did the author use "it" , "itself" rather than "he" and "himself" in here?

Question

How to understand the grammar in this sentence?
Here are the passages: "For more, check out Morgan’s work in full on the Internet Archive, which includes lovely passages like this one, marveling at how beavers build dams even though they don’t have to:

"As the dam is not an absolute necessity to the beaver for the maintenance of his life, his normal habitation being rather natural ponds and rivers, and burrows in their banks, it is, in itself considered, a remarkable fact that he should have voluntarily transferred himself, by means of dams and ponds of his own construction, from a natural to an artificial mode of life."

In the sentence: "it is, in itself considered, a remarkable fact that he should have voluntarily transferred himself,...", why did the author use "should have voluntarily" rather than "should voluntarily"? I mean "should have transferred" means the beaver didn't construct the dam, right? It's weird to me.  "it is, in itself condiered", why did the author use "it" , "itself" rather than "he" and "himself" in here?

Answer

This is a difficult sentence because of the style, but the grammar is not bad.  Taking the easy one first, Cory is right about "it" and "itself".

You obviously understand well the most common usage of should have + past participle.  The meaning here is different and formal. It does not have anything to with advice or obligation.  It is a formal way of saying : we expect that he has transferred himself.

It's an unusual construction and not one that you should spend a lot of time worrying about how to use in your own written English.  But you will see it from time to time in formal written English.

See Unit 20 of Advanced Grammar in Use by Hewings.

Answer

"should have" is a past model verb that when combined with a past participle (usually a verb in past tense) typically talks about things that did not happen but was a possible choice of action. Consequently, I agree that the text appears in error and should have read, " . . . it is, in itself considered, a remarkable fact that he voluntarily transferred himself, . . ."

As for the pronouns, "it" and "it is" is referring to the "remarkable fact", not the beaver.