Dead ball to live ball
Integration of black and foreign players
Lowering the pitching mound
Expansion of teams
Those are just a few off the top of my head. Hope that helps.
Video feedback to aid players
Train travel to air travel
Oops, this essay wasn't yours, Madison.
Well, anyway, that's a good 40 words or so.
Also - 100 years ago, many fans gambled openly in the stands, wagering on such minutia as the next pitch or the outcome of the next ball in play. Today, we have a version of that called "fantasy baseball."