To-night e To-morrow com hífen: Por quê?

"Uncle Pumblechook, being sensible that for anything we can tell, this boy's fortune may be made by his going to Miss Havisham's, has offered to take him into town to-night in his own chaise-cart, and to keep him to-night, and to take him with his own hands to Miss Havisham's to-morrow morning. And Lor-a-mussy me!" (Great Expectations, por Charles Dickens)

Existe diferença em relação as palavras tonight e tomorrow? Por que elas foram separadas por um hífen?

Thanks =)