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

The author of this letter concludes that a worldwide decline in the number of amphibians is an indication, or result, of global air and water pollution. To support this assertion the author first notes a decline in amphibians in Yosemite Park between 1915 and 1992, and acknowledges that trout, which eat amphibian eggs, were introduced there in 1925. But, the author then claims that the introduction of trout cannot be the reason for the decline in Yosemite because the introduction of trout in Yosemite does not explain the worldwide decline. I find this argument logically unconvincing in three critical respects.

First, the author fails to provide any evidence to refute the strong inference that the amphibian decline in Yosemite was indeed caused by trout. Because the author provides no affirmative evidence that pollution---or some other phenomenon--was instead the reason for the decline, the author's broad assertion that a worldwide decline in amphibians indicates global pollution is entirely unconvincing.

Secondly, even if I were to concede that the introduction of trout was not the cause of Yosemite's amphibian decline, the author provides no evidence that the decline was caused by pollution--rather than some other phenomenon. Perhaps some other environmental factor was instead the cause. Without ruling out all other possible explanations the author cannot convince me that pollution is the cause of the worldwide amphibian decline--or even the decline in Yosemite alone.

Thirdly', even if I were to concede that pollution caused Yosemite's amphibian decline, this single sample is insufficient to draw any general conclusion about the reason for a worldwide amphibian decline. It is entirely possible that the cause-and-effect relationships in Yosemite are not typical of the world in general. Without additional samples from diverse geographic locations, I cannot accept the author's sweeping generalization about the decline of amphibians and global pollution.

In sum, the scant evidence the author cites proves nothing about the reason for the general decline of amphibians worldwide; in fact, this evidence only serves to refute the author's own argument. To strengthen the argument the author should examine all changes occurring in Yosemite between 1915 and 1992 and show that air and water pollution have at least contributed to the park's amphibian decline. In any event, the author must provide data about amphibian population changes and pollution at diverse geographical locations; and this data must show a strong inverse correlation between levels of air and water pollution and amphibian populations worldwide.