Self-paced
Support Although most species of nondomestic mammals in Australia are marsupials, over 100 species— including seals, bats, and mice—are not marsupials but placentals. ██ ██ ██████ ████████ ████ █████ ██████████ ███ ███ ██████ ██ ████ ██████ ██████████ ███ ████████ ██████████ ██████ ███ ██████ █ ███ ██████████ ██ ███ █████ ██████ ████ ███████ ██████████ ███ ███████ █████ █████████ █████ ████ ████ ██████████ ████ ██ █████ ██ ██████████
Argument Summary
The author notes that while most nondomestic mammals in Australia are marsupials, there are over 100 species that are placentals. But because all nonhuman placentals in Australia—except the dingo, which was brought by the first humans who came to Australia—are descended from ancestors who could swim, fly, or float long distances, the author concludes that these placentals are not native to Australia.
Analysis: Sufficient Assumption
As usual, when we're looking for a sufficient assumption, it helps to trace the premise → conclusion structure of the argument. The main premise of the argument is that all of these placental mammals are descended from ancestors that were able to cross the ocean by swimming, flying, or floating (or, in the case of the dingo, by coming along with humans). The conclusion is that none of these placentals are native to Australia. In other words, this premise → conclusion structure maps out to:
ancestors swim/fly/float → /native to Australia
You might be tempted to make this assumption more abstract: e.g., because their ancestors could cross the ocean, they did cross the ocean. But you don't have to go that far to make sense of this sufficient assumption. You could also capture it by saying that no mammals that have been proven to be native to Australia had ancestors that could swim, fly, or float long distances, which is just the contrapositive of the structure above:
native to Australia → /ancestors swim/fly/float
So let's leave the diagramming as is and move on to the answer choices.
12.
The conclusion above is properly █████ ██ █████ ███ ██ ███ █████████ ██ ████████
a
Some marsupials now █████ ██ █████████ █████ ███ ██ ██████ ██ ████ ██████████ ███ ██████ █████ ████ ████ ██████████ ██ █████████ ██ ████ █████ ██████
Incorrect. We're trying to reach a conclusion about placentals not being native to Australia, not about marsupials being native to Australia or not.
b
Humans who settled █████████ ████████ ██████████ ████ ██ ███ █████████ ██████ ███████ ███ ███████ ██ ████ ██████████
Incorrect. Like (A), this answer choice tells us about marsupials. But the premise → conclusion gap we want to bridge is about placentals, not about marsupials.
c
The only Australian ██████████ ████ █████ ██ ██████ ██ █████████ █████ ██ ███████ █████ █████████ █████ ███ ████ ███████ █████████ ████ ██████████
Correct. (C) accurately bridges the premise → conclusion gap from the stimulus. Notice that (C) maps out to:
native to Australia → /could not reach Australia from elsewhere
By the contrapositive, this tells us that if a placental's ancestors could have reached Australia from elsewhere — e.g., by swimming, flying, or floating — then that placental cannot be native to Australia:
could reach Australia from elsewhere → /native to Australia
This perfectly bridges the gap we identified in our analysis.
d
No marsupials now █████ ██ █████████ ███ ████ ████ ██████████ ████ ██ █████ ██ ██████████
Incorrect. We're interested in placentals, not marsupials, and in the ancestors of current mammals, not in current mammals themselves.
e
Seals, bats, and ████ ███ █████████ █████ ████ ██ █████ █████ █████ ███ ██ ██████ ███████████
Incorrect. If (E) were true, this would only tell us that Australia's marsupials might also not be native to Australia. This doesn't help us draw the conclusion of the argument.