Family surnames can be a source of pride, dismay or confusion! We tend to focus on our own particular surname without realizing that, with almost every female ancestor we find, we add another family surname to our heritage. Let's look first at some of the problems with family surnames, then at their value.
The most common problem with family surnames can be the spelling. It seems to have been a mark of intelligence in previous centuries to see how many different ways a surname could be spelled. Indeed, people were often very creative in their spelling. In areas where your particular surname was prolific, variations in spelling can quickly lead you astray.
Another problem creeps in with translation at the time of immigration to make the immigrant appear more "American", or a customs office of one nationality tried to understand and write down the surname of a family of a different nationality. Schmidt and Smith are common examples and fairly obvious, but you might not think that Klein and Small are the same name unless you know German.
Along with this problem at the time of immigration is the census. Again, an enumerator familiar with one language may have stumbled over the surname of a family of a different nationality. Add to this the problem that indexers may have had in transcribing the name, and you might have to search for your family by given name, gender, age or even birth place!
A particular problem comes with your Irish ancestors. At times when it was desirable NOT to proclaim your Irish heritage, our ancestors often left the O' off their surnames. Thus O'Neill became simply Neil or Neal. Indexers of census records seem to have had a problem with this as well, and you may find your ancestor, Charles O'Neill, indexed as Neill, Charles O.
Let's look now at some of the advantages of family surnames. Today we often see children with a given or middle name that is, in fact, the surname of one of their parents or other ancestors. This can lead to confusion in the child's gender but can lead to their ancestors. In doing genealogical research, the same principle applies. Be aware, however, that your ancestor's given name may be the surname of someone other than his or her ancestor. I have seen a number of babies christened in a Scottish parish who were named for the minister - apparently a great favorite of the parishioners.
Family surnames can also give us a clue as to the occupation of our ancestors. For example, in my own line, John Baker married Jane Cooper. By the time this couple married in the early 1800s,, it is not known whether John was a baker, or whether Jane's father was a cooper (barrel-maker) but it is likely that their ancestors were employed in these occupations.
Many of us can trace our ancestors to Scandinavian countries where the naming pattern follows patronymics. In this pattern, a son would be named after his father. For example, Gustav, the son of Lars, would be Gustav Larson (or Gustav Lars' son). A daughter might be Kjersten Larsdottir (or Kjersten Lars' daughter). The custom of "dottir" for females did not survive as long as "son" or "sen" for sons. And, while you will know the given name of the father of your Scandinavian ancestor, you will not know his surname.
In summary, while family surnames can aid us in our research, you need to be aware of the pitfalls and snares that can accompany them. Be flexible in your approach and you are more likely to extend your family tree.