Are links an invention of the internet age?
When in the 1990s the Internet came up as a mass medium, philosophers were keen to theorize about ‘hypertext’. A hypertext exceeds the limitations of a linear text, it allows a reader to decide whether to go on reading the text or to follow a link to another text.
Nothing new under the sun. Marginalia and footnotes were the predecessors of such modern links, and they were used also in encyclopedias. According to Loveland, the Fons memorabilium by Domenico Bandini was one of the first works with cross referencing (ca. 1440). 
Not only, but especially in the age of modern encyclopedias with their alphabetical order those cross references were a means to cope with the mass of content.
Loveland noted a peticular problem about the Universal history (1740s): it had a lot of cross references, and they were usually accurate, but they referred only to whole articles, not parts of it.  This was especially problematic in a long-article-encyclopedia such as the Universal history. Spree wrote in 1999 about the uncertain consequences of new technologies for lexicography. Links to internet pages cannot guarantee their own continuity and longevity because content on the web is changing so quickly. 
In Wikipedia these two problems partly integrated to one double problem. In general, it is possible to link from one Wikipedia article to another and even to one specific article section. But Wikipedians recommend not to do so, because the name of a section can be changed, and then links to it will no longer work.
On the long term the problem vanishes. Many articles get more and more ‘sub articles’, meaning that new articles are created dealing with one aspect of a subject. For example, the article ‘Estonia’ in English Wikipedia was created in 2001; in February 2002 followed ‘Economy of Estonia’. So you don’t have to link to the ‘Economy’ section of the ‘Estonia’ article anymore.
 Jeff Loveland: An Alternative encyclopedia? Dennis de Coetlogon’s Universal history of arts and sciences (1745). Voltaire Foundation, Oxford 2010, p. 115.
 Jeff Loveland: An Alternative encyclopedia? Dennis de Coetlogon’s Universal history of arts and sciences (1745). Voltaire Foundation, Oxford 2010, p. 152.
 Ulrike Spree: Das Streben nach Wissen. Eine vergleichende Gattungsgeschichte der populären Enzyklopädie in Deutschland und Großbritannien im 19. Jahrhundert, Niemeyer 2000, p. 327.