Continuity of Self

Today is my birthday and it is fun to look at all of the greetings on Facebook from folks I've known over the entire range of my life.  Some who know me only as a graying department chair and others who knew me as a member of the red group in Wellwood Elementary, and every phase in between.  Seeing the mix of folks from all of the different places and stages and reflecting on who it is to whom they think they are sending birthday greetings, it does make you wonder if they are all going to the same person. 

If you look at the evolutionary development of a given type of animal, there are points along the line that we designate as different species.  They are related, but they are signified as different even though they are all points along the same developmental history.  In the same way, as we trace our own timelines, are there spots where we say "that was a different person"?  We have memories that span the time, more or less.  Does that give individuals a unity that species don't have?  Is it something else?  Am I the same person I was then -- whenever then happens to be?