The difference here lies in the meaning. You're basically asking why the descriptive phrase seems to be describing the second noun or object of the sentence--but in one case, it's used correctly and in the other case it's used incorrectly.
Here are the two examples:(1) "Fossils of the..sloth, made it the earliest known mammal..."
(2) "The intricate structure of the compound insect eye..."
The sentences seemingly have the same structure. Why, then is (1) no good and (2) is good?The difference is in the meaning. In (1)
when we say "fossils of the sloth" - we are bringing up two very different things. One is the actual sloth. The other is a fossil - something that was DERIVED from the original sloth.In (2)
when we say "intricate structure of the eye" - INTRICATE STRUCTURE is merely a DESCRIPTION of the eye. We're still talking about the eye, but more specifically we are referencing the intricate structure of the eye. You're still talking about the eye in general in both cases.
This contrasts with the situation in (1) where you are potentially referencing two different things - one is the fossil of the sloth and the other is the sloth itself. So when you use the word IT - the reader won't know which one you are referring to.
But in (2) when you use IT - you are clearly referencing the "intricate structure of the eye" as a whole mainly because the phrase is not composed of 2 different elements. Rather, the intricate structure is a characteristic of the eye.
Whereas the "fossil" is not really a characteristic of the "sloth."
For more information on these subtleties and other SC frameworks, please reference the SC Pill
Core Framework #3.
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