Logical Reasoning

PT141 · S2 · Q8 Biologist: Marine animals known

A free, expert breakdown of this official LSAT Logical Reasoning question.

Save & drill this skill — build targeted practice sets from questions like this one
Video walkthroughs — watch every question solved step by step
81 official LSATs — as questions, timed sections & full-length tests

Biologist: Marine animals known as box jellyfish have eyes with well-formed lenses capable of producing sharp images that reveal fine detail.

Conclusion (this shows that)

Eyes are adapted only to an animal's needs rather than to some abstract sense of how a good eye would be designed.

Evidence

Despite the fact that box jellyfish have eyes with well-formed lenses capable of fine detail, their retinas are too far forward to receive a clear image, so the jellyfish get only a blurry image that reveals prominent features but no fine detail.

Evaluate

This argument is presenting the somewhat curious fact that these jellyfish have lenses that could handle fine detail, but the positioning of their retinas only gives them a blurry image without fine detail.

The author might have been asking herself,

And her answer is, .

There are multiple New Concepts in the Conclusion, so we should take a sec and think about how they're intended to match up with the conversation.

"adapted only to its needs" is supposed to match up with the jellyfish's actual vision. So the author must be assuming that

is supposed to match up with the best case scenario for what this jellyfish's vision could be. So the author must be assuming that .

Goal

We have some clear Missing Links being assumed, based on these New Concepts in the conclusion.

Let's look for an answer defining what the author thinks "adapted only to an animal's needs" means, or what "how a good eye would be designed" means.

The argument requires assuming which one of the following?

Box jellyfish are the only
Too Strong
Too Strong: only Only Thing Mentioned ? Only Thing The author hasn't committed to this extreme claim that the only jellyfish with retinas that don't focus clearly are box jellyfish. The argument only mentions box jellyfish, but we're never accusing the author of assuming that the only X mentioned is therefore the only X. Otherwise, we'd accuse people who say "Black lives matter" of thinking that "Black lives are the only kind of lives that matter".
1% picked this
Correct
Box jellyfish have a need
Why this is right
This is one of the two missing links we predicted, based on these New Concepts in the conclusion. The author is concluding that the box jellyfish's vision is adapted only to its needs not to an ideal standard, since its vision can detect prominent features of objects in blurry focus but not in fine detail. So the author must be assuming that the box jellyfish's needs stop at "being able to see prominent features" and that's why it never evolved all the way to fine detail. If we negated this and said, "Box jellyfish have a need to detect prominent features and fine details" then that would utterly destroy the argument. There's no way we could conclude that its vision is adapted to its needs, if it needs fine details but its vision doesn't provide fine details.
Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.
91% picked this
Box jellyfish would benefit from
Out of Scope: benefit
Nothing in this argument is about whether or not a given thing would benefit an animal. If we negate this answer and say, "They wouldn't benefit from retinas that allow their eyes to focus more sharply", that wouldn't weaken the argument. In the author's mind, the fact that the box jellyfish's vision doesn't provide sharp focus shows that the box jellyfish doesn't need sharp focus. So if our negation were saying, "Hey, author -- the box jellyfish wouldn't even benefit from sharp focus", that wouldn't go against anything she's arguing. Whether we "need" something is a totally different inquiry than whether something would benefit us.
3% picked this
Box jellyfish developed from jellyfish
Opposite, if anything
The notion of what the box jellyfish evolved from is really out of scope. The author is just claiming that creatures evolve according to their needs, not according to ideal design. This answer is saying, "The author must assume that box jellyfish evolved from a species with more ideal design". That would actually seem more like a weakening idea than anything. The author thinks that the evolution of the box jellyfish's visual system stopped at "being able to make out prominent objects" and never evolved all the way to sharp focus / fine detail / clear images, because the box jellyfish didn't need vision that good. So if anything the author would probably think that the evolutionary predecessor of the box jellyfish had similar or worse vision.
1% picked this
Box jellyfish use vision as
Out of Scope
Out of Scope: detecting prey Too Strong: main We have no idea how the box jellyfish detects prey; we never talked about such a thing. There's no reason to think the author has committed to any idea about what is the box jellyfish's #1 means of detecting prey.
3% picked this

Continue the review in LSAT Lab

Save this question, watch the video walkthrough, and drill similar questions in your LSAT Lab account.

Save and practice this skill

LSAT Lab

Turn this review into a targeted study plan.

Save this question, drill more like it, watch the video walkthrough, and track your progress in your LSAT Lab account.

Start practicing free