What does Marks say would be the "Greatest modern imporvement" he can think of?

That would be 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."