Any One Out There That Can Answer This Question Please

I have a Eugene Munro born 1885 Melbourne Victoria. Mother Christina Munro, father unknown. In 1888 Christina Munro marries William James. From then on Eugene takes the name of James eg. 1909 marries as Eugene James. Would he have to have been adopted or had his named changed leagaly. Or back then was it just usual to have done this. If so how would he have been married if his birth certificate is of a different surname? Or did they not have to produce a birth certificate on marriage. Could someone please throw some light onto this for me. Many thanks to you all for taking the time to read this.
Best Wishes Dide