"With the practice of the color line, you needed someone who was unambiguously black to take the color line and dismantle it," Burgos said. "Jackie had to take it all, that weight and that burden."
The key word there is "unambiguously." Through research, Burgos documented numerous cases of Latinos who could be brought into organized baseball not because they were white, but because they were not black.
These were Cuban players of Spanish descent, and that racial connection was critical if people were going to accept the players.
In the 1908 Connecticut League, New Britain had four Cuban players on its team. In response to protest, the league enacted a policy preventing teams from signing "black" players.
As Burgos wrote, because the Connecticut League didn't have African-American players, the Cuban players' racial ancestry moved to the forefront.
The ruling sent New Britain general manager Billy Hanna into a scramble to confirm that his four Cuban stars were eligible under the new rule. Hanna traveled to Cuba and returned with verification for three of the four. Luis Padron -- who was fourth in the league in hitting and one of New Britain's best pitchers -- could not be cleared and had to leave the team.
Much of ensuring players could pass as non-black Cubans and stay in organized ball hinged on paperwork and, in some cases, media spin. Exotic was OK as long as the blood could be traced to some sort of European origin, whether Castilian or Portuguese.
These distinctions led to a stratification of the color line. Brownfolk and players of Native American descent could gain some inclusion, but the color line stopped well ahead of black players.
"These owners were quite sophisticated in terms of how to operate this system," said Burgos, a history professor at the University of Illinois, Urbana-Champaign. "They created 'brown' and 'red' categories. African-Americans and players of African descent from the Caribbean could not be presented in this racially ambiguous way.
"With Roberto Estalella of the Washington Senators, they said, 'He's not black, he's Cuban.' Cuban-ness would present him differently in the eyes of some, but in reality, some teammates viewed him as black and treated him as so."
In his book, Burgos does a thorough job of describing this system of skirting the color line, as well as its effects. One group the system was not lost on was African-Americans. In the years leading up to 1947, the black media questioned how it was possible to manipulate the color line so that certain players could be included because of their "non-black" status and criticized the inconsistency of signing foreign-born Latinos while excluding blacks.
"Black sports writers commented on it continuously over the years," Burgos said. "'How is it that these darker skinned players, who may be called mixed or mulatto or caramel-colored Cubans, can be included when [half-African-American, half-Italian player] Roy Campanella, who had lighter skin, did not?' That was why Jackie had to take on [the burden of breaking the color barrier]."