By 1900, the philatelic world was already too vast for anyone to complete. Their first specialty albums began to appear about 1920, and it was these albums that drove the push towards increasing specialization. Until about 1960, nearly everyone entering the hobby began a general collection. Packets and general albums which had spaces in them for the stamps that were in the packets were everyone's way of beginning collecting. But today, there are just too many stamps that have been issued for any but the hardy to try to collect all of them.
Specialization began as a way of managing the collecting habit. Instead of the million or so different stamps that are listed by the catalogs, the US specialist needs only 4,000 or so if he collects regular issues or 10,000 or so if he wants all the US subspecialties. But even this has proven too daunting, and many collectors, especially in the last fifty years, have become increasingly specialized in narrower and narrower subsections of main countries. Collectors of United States stamps now will specialize in Washington-Franklin Offset issues or Revenues or really any of hundreds of subspecialties that line the US collecting landscape alone (not to mention covers and postal history). The increasing fineness of focus is an individual decision, but it has had ramifications for our hobby that have not always tended to increase the appeal of our hobby in general.
The number of philatelic subspecialties is staggering. Thousands of philatelic books have been written on tiny specialized areas from subjects as diverse as Australian Aerograms to Norwegian steamship cancellations. And add to these thousands of books, tens of thousands of philatelic articles in the periodical press on subjects that are often as vast, collectible, and significant as those that have had full books written on them. The amount of knowledge in our hobby is incredible. I have written of going through hundreds of cartons of pre-1900 philatelic literature in the Philadelphia Library that is not even cataloged. Indeed, I think that stamp collectors can have a special understanding of the vastness of the knowledge in the world and the things that there are to learn and understand. If what there is to know in our hobby is so great that in ten lifetimes of constant study no one could get all of it, then how incredibly vast must be the sum of human knowledge in all the hundreds of thousands of non-philatelic subjects, many of which have far more to know that we have in stamps?
Specialists sometimes forget that they didn't begin in the hobby by trying to find out all that there was to know about the advertising back prints of New Zealand or compound perforations on Romanian postage dues. They made a decision to spend their time learning this, and in doing so they made a decision not to have the time to learn other philatelic things. When I was in college, I took a course given by a professor who felt that no other aspect of history, from the beginning of the earth to the latest election, was worth knowing except for the wonders of the tobacco planters who created the Virginia Colony in the 1600s. It was then that I saw that Isaiah Berlin was probably right about learners coming in two main types-foxes, who know many things and hedgehogs who know one thing well. Hedgehogs write more articles and books while foxes, who spend as much time with their interests, may know as much or more in total, but tend to do so in a quieter way.