Bill Poser at Language Log has a wonderful post laying out the basics of historical linguistics that should be required reading for anyone even thinking about pontificating on the field, and it should be memorized by anyone actually working in it. I want to single out here a quote from Georg von der Gabelentz’s Die Sprachwissenschaft (1901) which is as pertinent today as it was when he wrote it:
It is terribly seductive to roam the world of languages comparing words from them at random and then to bestow upon scholarship a series of newly discovered relationships. Very many stupidities also result from this; for the most urgent discoverers have unmethodical minds. He who, endowed with a good memory for words, has gone through a couple of dozen languages from different parts of the Earth, – he need not at all have studied them -, finds familiar forms everywhere. And if he records them, investigates them, tests intelligently whether the indications pan out, he does only what is right. Only logically correct thought belongs here, and where it is not absent from the outset then he gladly gets lost in the giddiness of the mania of discovery. Thus it went, as we saw, with the great Bopp, when he sought to assign Caucasian and Malayan languages to the Indo-European language family. Fortune had decreed him a curious fate. It was, to have to prove the correctness of his principles twice, first positively through his magnificent main work, which is based on them, then, negatively, by coming to grief as soon as he was unfaithful to them… Languages are different because sound change has taken different paths. But it has gone its way consistently hither and thither; therefore Order reigns in differentiation, not Chaos. Language comparison without comparison of sounds is irresponsible game-playing.
Poser provides the German original, and also links to a pdf file of his and Lyle Campbell’s paper “Indo-European Practice and Historical Methodology,” which I commend to your attention if you’re interested in more details. The fight against sloppiness is endless but must continually be fought.