Fairly early in the 19th century, the idea that languages were a type of natural organism became popular among German scholars of language. Thus, it is not that surprising that soon after the publication of Darwin's Origin of Species in 1859, the German philologist August Schleicher (1821-1868) made the argument that "linguistics should be regarded as one of the natural sciences to which Darwin's theory applies" (Sampson, p. 18). As Sampson tells us, Schleicher's perspective was that
The linguist's language families, languages, dialects and ideolects [the form of a language spoken by an individual] correspond to the biologist's genera, species, varieties, and individuals. Languages and language-families, like species, compete with one another in a "struggle for survival" (consider, in the British Isles for instance, how English has spread at the expense of the Celtic languages [like Welsh]...); and, on a world scale, Schleicher saw the Indo-European language-family as having reached a dominant position linguistically, as Man has become the dominant zoologically. (pp. 18-19)
Furthermore, some languages, like Etruscan and Gothic, even become extinct, just like some biological species do (e.g., dinosaurs and mammoths).
Given these apparent parallels between languages and biological organisms (as well as the intellectual prestige of Darwinism at the time), it is not surprising that a view like Schleicher's could be easily accepted by linguists. Nevertheless, by the end of the 19th century, the idea that human languages had developed via a process akin to Darwinian evolution had fallen into disrepute, for several reasons. For one thing, while Darwinian evolution assumes that biological changes occur in a particular direction--from simple to complex, from "lower" to "higher"--changes in languages do not exhibit a consistent direction over time. In addition:
If one gives up the idea that language change regularly proceeds in a particular direction, it become difficult to follow Schleicher in applying to language Darwin's concepts of "natural selection" and "struggle for survival": what, in language, will correspond to the biological notion of aptitude for survival? And in fact the expansion of certain languages at the expense of others seems to be explainable very adequately in terms of social factors, so there is no room for an explanation referring to the intrinsic merits of the languages themselves. It might be perhaps be that English is in some sense a "simpler" or "more advanced" language than Welsh; but the fact that English has been expanding and Welsh contracting is undoubtedly due to the fact that England has been a centre of power and wealth and Wales has not. (Sampson, p. 26)
Finally, as Sampson further points out, "another real problem for the evolutionist view of language had to do with the causation of changes" (p. 28--emphasis mine). This was especially true with regard to changes in the pronunciation of words over time. An example of such a change would be the so-called Great Vowel Shift that occurred between the fifteenth and eighteenth centuries in English. As a result of the Great Vowel Shift, certain vowels in English came to be produced higher in the mouth, and thus the pronunciation of certain words changed (for instance, the word life was pronounced more like the word leaf prior to the Great Vowel Shift). However, there is no clear explanation as to why this happened. Indeed, despite much theorizing in the 19th century, philologists were never able to provide a convincing explanation as to why sound-changes like the Great Vowel Shift happen.
Consequently, in light of the reasons discussed above (and perhaps for other reasons), the notion that languages have evolved just like biological species have (supposedly) evolved was rejected by most linguists. Nevertheless, most linguists still believe that language and evolution are connected. Nowadays, the development of human language is viewed as a product of evolution itself, rather than being an analogue to biological evolution. Whether such a belief is well-founded, is, however, another matter, as I have and will continue to argue in this blog.
Image of August Schleicher from Wikipedia