What is the greatest mod'rn improvement Haley think of concerning slaves?

The greastest modern improvement would be to find slave women who didn't care about their children.

"If we could get a breed of gals that didn't care, now, for their young uns," said Marks; "tell ye, I think 't would be 'bout the greatest mod'rn improvement I knows on," -- and Marks patronized his joke by a quiet introductory sniggle."