Home runs existed before outfield fences did. A player simply ran around the bases before the ball could be thrown home. Home runs became more special, though, when walls were built around ballfields in the late 1800s. (Historians can't pinpoint the date, as photos of ballparks then are scarce.)
The fences were not to keep baseballs in, but to keep nonpaying spectators out. Baseball was growing in popularity.
The National League was formed in 1876. That year, Ross Barnes of the Chicago White Stockings slugged the first big-league homer. Only 40 home runs total were hit in the 260 pro games played that year. (Part of the problem was that in parks with shallow fences, a baseball hit fair over a shallow fence was ruled a double.)
In 1902, Tommy Leach led the National League with just six homers. That was the depth of the so-called "dead ball" era. Things changed dramatically in the 1920s, with Babe Ruth and the advent of a baseball that many thought was livelier. The so-called "rabbit ball" theory is disputed by historian William Curran. He says that the hitting revolution was fueled by Ruth's more free-swinging hitting style, the outlawing of the spitball, and the use of clean, more easily visible baseballs after a fatal beaning of Ray Chapman in 1920.
Ruth was the game's original slugging superstar. In 1919 he belted a record 29 homers, while his Boston Red Sox teammates hit only four. After being traded to the Yankees for the 1920 season, Ruth hit an eye-popping 54 home runs. The Yankees (to the chagrin of the Red Sox) set a new major-league attendance record: 1.3 million fans. That was four times their 1917 home attendance.
When Ruth retired in 1935, he held two major records: most homers in a single season (60 in 1927) and most career home runs (714).
Roger Maris of the Yankees eclipsed Ruth's single-season mark in 1961 with 61 homers. Hank Aaron of the Atlanta Braves surpassed Ruth's career milestone in 1974 on his way to a total of 755.
Power hitting has increased in recent years. Why? Experts point to stronger players, a dilution of pitching talent because of so many more teams, and homer-friendly ballparks like mile-high Coors Field in Denver.
Top Single-Season Home-Run Hitters*
1. Mark McGwire, St. Louis Cardinals (1998) 65
2. Sammy Sosa, Chicago Cubs (1998) 63
3. Roger Maris, New York Yankees (1961) 61
4. Babe Ruth, New York Yankees (1927) 60
5. Babe Ruth, New York Yankees (1921) 59
6. Jimmie Foxx, Philadelphia Athletics (1932) 58
(tie) Hank Greenberg, Detroit Tigers, (1938) 58
(tie) Mark McGwire, Oakland A's and
St. Louis Cardinals (1997) 58
9. Hack Wilson, Chicago Cubs (1930) 56
(tie) Ken Griffey Jr, Seattle Mariners (1997) 56
*through Sept. 20, 1998