what is a chemical found in tobacco smoke that speeds up the activities of the nervous system, heart and other organs; it produces the addiction to tobacco smoke.

Nicotine is the addictive chemical in tobacco

The person who utilize the progression principle will keep adding the difficulty of the exercise that he's doing, while the person who does not, will only stay stagnant.  the person that utilize progression principle, will get a stronger endurance because now his body is conditioned to do harsher routines.{the answer was already on verified)