In regard to Percival Lowell's claims of a vast canal system on Mars, Alfred Russel Wallace wrote "[a network of canals,] as Mr. Lowell describes, would be the work of a body of madmen rather than of intelligent beings." Why?

Because according to Alfred Russel Wallace, the martian surface was cold, and the atmospheric pressure was too low to allow liquid water. All these features made it impossible for the existence of intelligent life, and consequently, the canals advocated by Percival Lowell.

Explanation:

The idea of intelligent life on Mars was a profitable argument in the late 19th century. Because of the lack of technology and the impossibility to see the martian surface with precise tools, many speculations about the Red Planet's surface were popular. In 1877 astronomer Giovanni Schiaparelli said that Mars was covered by canals (canali), an argument resulted from his observation during a closer transit of Mars from Earth. The idea, however, would be popularized by Percival Lowell and his book "Mars", originally published in 1895. According to Lowell, the martian canals were resulted by modern engineering, performed by superior intellectual beings. However, the idea, even if it's creative and spread for several other media, especially novels, it was subsequently dismissed by scientists.