another question

Can someone tell me the meaning of the following sentence? "Horse-drawn sledges are the connecting links between ocean commerce and New England farms. " This sentence was used to describe Boston, USA in 19th century.
"commerce" is an abstract word, while "farm" is a concret word. I wonder why these two words are linked by "between... and...".
Thank you very much.

The commerce was goods that were traded at a seaport. It could have been from seafood captured off the coast and/or goods imported by ship to the American coast. From there, the goods were transported to New England farms by horse-drawn sledges.