#12 - Although most species of nondomestic mammals in

[Administrator]

Complete Question Explanation

Justify the Conclusion. The correct answer choice is (C)

The argument is that since the placental non-domestic animals in Australia, other than the dingo, could swim, fly, or float, those animals did not come from Australia.

Of course the argument seems ludicrous. Just because these animals could swim, fly, or float to Australia does not mean they came from somewhere else. However, we are asked to justify the conclusion.

Answer choice (A): Since this choice is about marsupials rather than placentals, it is irrelevant, and wrong. Furthermore, it does not justify by analogy, since it establishes only possibility, not certainty.

Answer choice (B): This answer choice is about what probably happened most of the time, and is not sufficient to conclude what definitely happened all of the time.

Answer choice (C): This is the correct answer choice. If the only way a placental could be native would be that it could not come from elsewhere, we have to conclude that the domestic placentals in Australia came from elsewhere. We could make the following conditional diagram:

• Premise: Placentals in Australia → Can come from elsewhere.
  
  Answer choice (C): Can be native → Cannot be from elsewhere.
  
  contra positive: Can be from elsewhere → Cannot be native.
  
  Conclusion: Placentals in Australia → Cannot be native.

Answer choice (D): Marsupials are irrelevant to the issue, and this choice is wrong. A dissimilarity between marsupials and placentals does not make the argument's conclusion better.

Answer choice (E): This answer choice is about what is "typically" the case, and does not establish a conclusion about what is always the case.

[AnnBar]

Hello,

I am having difficulty understanding why C is correct.

All placentals in A (except Dingo) --> ancestors could F/F/D
Placentals--> NOT native

Answer C: only placentals in A that could be native would be animal whose ancestors could not have reached A from elsewhere (meaning NOT F/F/D)

I am confused as to how the "ALL placentals" then allows for answer choice C because it seems to contradict itself, saying that other placentals could have reached A without F/F/D.
I am confusing myself as I am writing this. I hope it makes sense.
Thank you

[Ricky_Hutchens]

Hi AnnBar,

Let's break it down.

All P's in Australia can F/F/D.
Missing Step
All P's in Australia must not be native.

The question is what is the missing step. C says that only animals that can't F/F/D can be native to Australia. If you pop that in above, the argument suddenly makes sense.

[ccanno]

Hi,

I still don't understand how C is the correct answer. If all Placenteals are not native, then why does "could not have reached Australia from elsewhere" help the argument. Wouldn't something that could come over "swim long distances, fly, float..." be ways they could come from elsewhere?

Thanks

[KelseyWoods]

Hi ccanno!

It helps to treat these Justify questions very mechanistically. Focus on the specific wording of the conclusion that you are trying to prove. And find an answer choice that if added to the premises will prove the conclusion 100%. Here's the equation we're trying to solve:

Premise: All nonhuman placentals had animal ancestors that could swim long distances, fly, or float on driftwood
+
[Correct Answer]
=
Conclusion: Placentals are not native to the island continent

We're trying to prove that placentals are not native to the island based on the fact that they had ancestors who could travel far distances. Essentially we're looking for a link between that premise and the conclusion--something that tells us that if that premise is true, then that conclusion is true. In this case, we need something that tells us that if the placentals had animal ancestors that could travel long distances, then placentals are not native to Australia.

Answer choice (C) states: "The only Australian placentals that could be native to Australia would be animals whose ancestors could not have reached Australia from elsewhere."

That is a conditional statement and, thus, we can diagram it:

Placentals native to Australia Ancestors could have reached Australia from elsewhere

As written, this doesn't appear to do much for us because it's telling us what must be true if placentals are native to Australia and we're trying to prove that the placentals are not native to Australia. But the contrapositive of this statement may help us out:

Ancestors could have reached Australia from elsewhere Placentals native to Australia

That matches our prephrase and links the premise to the conclusion. We know that the placentals had animal ancestors that could have reached Australia from elsewhere (because they could travel long distances). If the ability to come from elsewhere is enough to prove that placentals are not native to Australia (as answer choice (C) states it is), then that proves the conclusion that the placentals are not native to Australia.

Premise: All nonhuman placentals had animal ancestors that could swim long distances, fly, or float on driftwood
+
Answer Choice: Ancestors could have reached Australia from elsewhere Placentals native to Australia
=
Conclusion: Placentals are not native to the island continent

Hope this helps!

Best,
Kelsey

[g_lawyered]

Hi Kelsey,
I have a question regarding your detailed explanation. Here's how you broke it down:

Premise: All nonhuman placentals had animal ancestors that could swim long distances, fly, or float on driftwood
+
[Correct Answer]
=
Conclusion: Placentals are not native to the island continent

We're trying to prove that placentals are not native to the island based on the fact that they had ancestors who could travel far distances. Essentially we're looking for a link between that premise and the conclusion--something that tells us that if that premise is true, then that conclusion is true.

THIS IS WHERE MY CONFUSION BEGINS:
In this case, we need something that tells us that if the placentals had animal ancestors that could travel long distances, then placentals are not native to Australia.

Why do we need to assume that (your explanation above ^ ) to prove the conclusion? I thought that is what the premise is saying exactly-- that is: BECAUSE the animals had ancestors that could swim long distances, fly, and float on drift wood is THE REASON WHY the author concludes that the animals aren't native to Australia. This means that, in order for those animals to get to Australia they must've done 1 of the 3 (either swam, flew, or floated). I was under the impression that I needed to justify that. I thought that I needed an answer that justified- that is how the animals got to Australia in the first place (that there isn't another way the animals could've migrated to Australia).
Can someone explain to me where my reasoning went wrong?

I realized my approach is different from the explanation, in that I Didn't ID nor write out the premise in conditional reasoning format, nor did I write out correct answer choice C in conditional reasoning format (nor it's contrapositive) and that is the reason why I didn't choose C.

Instead, I attempted to ID new concept in the argument and use it to come up with the assumption. I thought that the wording in the premise of: " swim long distances, fly, and float on driftwood" was a new concept. I attempted to use that along with the "native" concept in the conclusion. For this reason, I chose answer choice D. I also chose D because it mentions a difference between marsupials and placentals.
Why is answer choice D incorrect?

I appreciate any guidance in my great confusion with this question. Also, I'm concerned that the level of difficulty on P.S. homework isn't as high as the level of difficulty I found the question to be.

Thanks in advance!

[Rachael Wilkenfeld]

Hi GGIBA003,

I think you have yourself turned around a bit. Think of the premises as what we know, and the conclusion as what we are trying to prove.

We know these animals' ancestors could s/f/f. We want to be able to draw the conclusion that the placentals are not native to Australia.

Be careful, because you are filling in the missing blanks in the argument for the author. You can't do that. You can't fill in what they did not say.

We want to justify that conclusion that these placentals aren't native. So we need something that says if you could s/f/f, then you aren't native. Or if you are native, then your ancestors can't s/f/f. That's what we see in answer choice (C), the contrapositive of the link between the premise (sufficient) and the conclusion (necessary).

For answer choice (D), it doesn't tell us anything about the placentals, which is what we are trying to prove. It doesn't get us to the conclusion given in the stimulus.

Hope that helps!