Support Beam Splitting
This is not meant to be an authoritative answer, but I have worked in many homes and buildings of similar age and you almost always see some cracks in the beams, they are usually there for decades, in fact they probably occur when wood is new and drying out for the first time.  

Still it can't hurt to have someone take a look at it.  You could take some measurements and recheck it periodically, see if it is sagging or the crack is getting bigger.  If it isn't, it may be of no concern.  If necessary, they may be able to fill the crack with epoxy and / or sister on some lumber to stiffen it.