"But let's imagine what it would be like if substitution did develop in cycling like it did with almost every other sport. The first step in imagining that is to recognize that substitution wouldn't come into play so much in the one-day races. Its hard to imagine rules where a rider would stop midway through a race and another rider take its place. Only a couple sports have substituting on the fly like that. Besides, Specialization (proto-substitution) has already fixed this problem for the most part. The riders starting a one day race are tuned to the demands of that race.  No, specialization would be mainly in stage races-which is why I am writing this now and not during cobbles season. The Tour de France would be the first place to implement substitution." - Chester