The Galápagos finches have fascinated evolutionary biologists for nearly two centuries. _____ by Charles Darwin during his 1835 voyage aboard...

Sentence Structure

• The Galápagos finches
  • have fascinated evolutionary biologists
    • for nearly two centuries.

• (?) by Charles Darwin
  • during his 1835 voyage aboard the HMS Beagle,
• the birds
  • provided crucial evidence
    • for the theory of evolution through natural selection.

Understanding the Meaning

The first sentence is straightforward:

• The Galápagos finches have fascinated evolutionary biologists for nearly two centuries.
  • These particular finches have been really interesting to scientists who study evolution for almost 200 years.

Now the second sentence begins:

• '(?) by Charles Darwin during his 1835 voyage aboard the HMS Beagle'
  • This is where we have the blank.

Let's look at our choices:

• Observes, Observed, Observing, Observe
  • These are all different forms of the same verb.
  • We're deciding what form works here.

To see what works here, let's read the rest of the sentence and understand what it's saying:

• 'the birds provided crucial evidence for the theory of evolution through natural selection.'
  • So "the birds" is the main subject
  • and "provided" is the main action of this sentence.

Now let's understand what this is telling us:

• The opening phrase '(?) by Charles Darwin during his 1835 voyage aboard the HMS Beagle' comes before the main subject and verb
  • This phrase is describing something about "the birds"
  • It's giving us background information about them

• The key signal here is 'by Charles Darwin'
  • When we say something was done "by" someone, we're describing an action that was done TO something
  • Darwin did the action TO the birds - he observed them
  • The birds received the action - they were observed by Darwin

So the complete picture is:

• The birds, having been observed by Darwin in 1835, went on to provide crucial evidence for evolution

What do we notice about the structure here?

• We need a verb form that shows the birds were on the receiving end of the action
  • The birds didn't observe - they WERE observed
  • This is a passive meaning: something was done TO them BY Darwin

• To create this passive meaning in a descriptive phrase, we need the past participle form: "Observed"
  • "Observed by Charles Darwin... the birds provided..."
  • This works perfectly to show that Darwin observed them

The correct answer is B. Observed

GRAMMAR CONCEPT APPLIED

Using Passive Voice in Descriptive Phrases

When you start a sentence with a phrase that describes your subject, and that description involves an action done TO the subject BY someone else, you need the past participle form (called passive voice in grammar terms) to show this relationship:

Pattern: [Past Participle] by [agent], [subject] [main verb]...

Example 1:

• Active: "Darwin observed the birds during his voyage."
  • Darwin = subject doing the action
• Passive descriptive phrase: "Observed by Darwin during his voyage, the birds provided evidence..."
  • Birds = subject receiving the action (they were observed)
  • "Observed" = past participle showing passive meaning

Example 2:

• Active: "The committee selected these candidates."
• Passive descriptive phrase: "Selected by the committee, these candidates will advance to the finals."
  • "Selected" = past participle showing the candidates received the action

In our question:

• The birds didn't do the observing - they WERE observed
• "by Charles Darwin" signals that Darwin was the agent doing the action TO the birds
• Therefore, we need "Observed" (past participle) to create this passive meaning
• "Observed by Charles Darwin... the birds provided..." ✓