Final answer:
John Thornton warns against traveling to Dawson due to the extreme and perilous weather conditions of the Yukon, which could be fatal for the unprepared travelers.
Step-by-step explanation:
John Thornton advises Charles, Hal, and Mercedes not to go on to Dawson due to unfavorable weather conditions. This warning comes from the book 'The Call of the Wild' by Jack London, in which Thornton understands the extreme dangers posed by the harsh and unforgiving Yukon environment, particularly the cold and the threats it poses to travelers who are not well prepared. His advice stems from the perilous nature of the journey in such severe weather conditions, which could lead to potentially fatal consequences for the inexperienced party.