Historical linguistics is the study of how a language develops into its present form. This can include changes from older versions of the language in speech and grammar, as well as the etymology of loanwords and neologisms.
History of Historical Linguistics
For a long time, people knew that languages could evolve into new languages, based on the development of Latin into Vulgate Latin, and thence into the Romance languages. However the field of historical linguistics is relatively new, dating back to the 19th century.
Traditionally historical linguistics has attempted to reconstruct ancestor languages via the process of the comparative method. The comparative method attempts to find regular sound changes between languages.
A new method of historical linguistics was developed by Joseph Greenburg, called mass comparison. The method remains controversial. Most of the language families Greenberg constructed are not accepted by the majority of linguists, however Afro-Asiatic and Niger-Congo have been almost wholly accepted, and Nilo-Saharan has been accepted with some reservations, many linguists splitting off groups.
The principle of mass comparison is that languages generally don't borrow very basic vocabulary, such as pronouns, though there are examples of borrowing in mixed languages. As such, he looks for general similarities, which supposing that words can have semantic shift.