The following is a letter to the editor of an environmental magazine
"The decline in the numbers of amphibians worldwide clearly indicates the global pollution of water and air. Two studies of amphibians in Yosemite National Park in California confirm my conclusion. In 1915 there were seven species of amphibians in the park, and there were abundant numbers of each species. However, in 1992 there were only four species of amphibians observed in the park, and the numbers of each species were drastically reduced. The decline in Yosemite has been blamed on the introduction of trout into the park’s waters, which began in 1920 (trout are known to eat amphibian eggs). But the introduction of trout cannot be the real reason for the Yosemite decline because it does not explain the worldwide decline."
In this argument, the writer of the letter concludes that global pollution of water and air has caused a decline in the number of amphibians worldwide. To support his or her conclusion, the writer cites the results of two studies, seventy-five years apart, that purportedly show that the number of amphibians in one park in California, Yosemite National Park, have drastically declined. Additionally, the writer casts aside a given reason for the decline, stating that the introduction of trout to the park (who are known to eat amphibian eggs) does not explain the worldwide decline in the number of amphibians. This argument defies simple logic and suffers from several critical fallacies.
First of all, the argument is based on only two studies in one specific part of the world, Yosemite National Park in California. It is impossible to pinpoint a worldwide theory for the decline of amphibians based on any number of studies in only one specific location in the world - the specific varieties of amphibians, geographical conditions and other location specific variables prohibit such a sweeping generalization. One very specific location cannot be used as a model for all other locations, even within one particular country, let alone the entire world. The writer provides no evidenced whatsoever that links the Yosemite study with any purported effects anywhere else in the global environment.
Secondly, the two separate studies were done seventy-five years apart. There is no evidence that the two studies were conducted in a similar manner over the same duration of time or even over the same exact areas of Yosemite National Park, or that the exact same study methods were used. For example, perhaps the first study lasted over an entire year and was conducted by twenty-five experts in amphibious biology, resulting in the finding of seven species of amphibians in abundant numbers. By contrast, perhaps the second study was conducted over a period of one week by a lone high school student as a school science project. The writer offers no basis on which to compare the two studies, leaving it open as to whether the two are truly comparable in their breadth, scope and expertise.
Finally, the writer notes that the decline in the amphibian population has been blamed on the introduction of trout into the park’s waters in 1920, but then dismisses that argument on the purely specious basis that it does not explain the worldwide decline. This part of the argument blithely dismisses the very relevant fact that trout are known to eat amphibian eggs. This attempt to "prove a negative" is the last resort of those in search of some vain attempt to prove the truth of the matter that they are asserting. It is basically impossible to "prove a negative"; this is an attempt to shift the burden of proof back on to the nonbelievers of the argument. The global environmental situation and that of Yosemite National Park are not perfectly correlated, and the fact that the trout may very well be responsible for the decline cannot simply be dismissed without further proof.
In summary, the writer fails to establish any causal relationship between global air and water pollution and the decline of amphibious life worldwide. The evidence presented is extremely weak at best and narrowly focuses on one tiny area of the globe, as well as putting forward as proof two studies about which almost nothing is known. For a stronger argument, the writer would need to directly put forth evidence associating air and water pollution with not only the decline at Yosemite but also throughout other areas of the world.
最后，信函作者指出，两栖动物种群数量的减少，已被人归咎于1920年将鲑鱼引入公园水域这一做法，但紧接着又以该论据无法解释世界范围内动物数量减少这一似是而非的依据将该论据予以否认。信函作者论述中的这一部分漫不经心地将一个极为相关的事实弃置不顾，即众所周知，鲑鱼喜食两栖动物所产的卵。这种"prove a negative "的尝试往往是这样一类人所惯用的最后伎俩，他们竭力寻找某种徒劳的尝试，力图去证明他们所宣称的事物的真理。从根本上讲，"prove a negative"是不可能的。这样一种做法是试图将论证的负担重新转嫁给不相信该论据的人。全球的环境情形与约塞米蒂公园的情形并不绝然对应。鲑鱼极有可能造成了两栖动物数量减少这一事实在缺乏进一步证据的情况下是断不能轻易予以否认的。