The line of reasoning which the Romance philologist follows in his study of vulgar Latin is equally convincing. The existence of a large number of words and idioms in French, Spanish, Italian, and the other Romance languages can be explained only in one of three ways. All these different languages may have hit on the same word or phrase to express an idea, or these words and idioms may have been borrowed from one language by the others, or they may come from a common origin. The first hypothesis is unthinkable. The second is almost as impossible. Undoubtedly French, for instance, borrowed some words from Spanish, and Spanish from Portuguese. It would be conceivable that a few words originating in Spain should pass into France, and thence into Italy, but it is quite beyond belief that the large element which the languages from Spain to Roumania have in common should have passed by borrowing over such a wide territory. It is clear that this common element is inherited from Latin, out of which all the Romance languages are derived. Out of the words, endings, idioms, and constructions which French, Spanish, Italian, and the other tongues of southern Europe have in common, it would be possible, within certain limits, to reconstruct the parent speech, but fortunately we are not limited to this material alone. At this point the Latinist and the Romance philologist join hands. To take up again the illustration already used, the student of the Romance languages finds the word for "horse" in Italian is cavallo, in Spanish caballo, in French cheval, in Roumanian cal, and so on. Evidently all these forms have come from caballus, which the Latinist finds belongs to the vocabulary of vulgar, not of formal, Latin. This one illustration out of many not only discloses the fact that the Romance languages are to be connected with colloquial rather than with literary Latin, but it also shows how the line of investigation opened by Diez, and that followed by Woelfflin and his school, supplement each other. By the use of the methods which these two scholars introduced, a large amount of material bearing on the subject under discussion has been collected and classified, and the characteristic features of the Latin of the common people have been determined. It has been found that five or six different and independent kinds of evidence may be used in reconstructing this form of speech.
We naturally think first of the direct statements made by Latin writers. These are to be found in the writings of Cicero, Quintilian, Seneca the Rhetorician, Petronius, Aulus Gellius, Vitruvius, and the Latin grammarians. The professional teacher Quintilian is shocked at the illiterate speech of the spectators in the theatres and circus. Similarly a character in Petronius utters a warning against the words such people use. Cicero openly delights in using every-day Latin in his familiar letters, while the architect Vitruvius expresses the anxious fear that he may not be following the accepted rules of grammar. As we have noticed above, a great deal of material showing the differences between formal and colloquial Latin which these writers have in mind, may be obtained by comparing, for instance, the Letters of Cicero with his rhetorical works, or Seneca's satirical skit on the Emperor Claudius with his philosophical writings. Now and then, too, a serious writer has occasion to use a bit of popular Latin, but he conveniently labels it for us with an apologetic phrase. Thus even St. Jerome, in his commentary on the Epistle to the Ephesians, says: "Don't look a gift horse in the mouth, as the vulgar proverb has it." To the ancient grammarians the "mistakes" and vulgarisms of popular speech were abhorrent, and they have fortunately branded lists of words and expressions which are not to be used by cultivated people. The evidence which may be had from the Romance languages, supplemented by Latin, not only contributes to our knowledge of the vocabulary of vulgar Latin, but it also shows us many common idioms and constructions which that form of speech had. Thus, "I will sing" in Italian is cantero (=cantar[e]-ho), in Spanish, cantare (=cantar-he), in French, chanterai (=chanter-ai), and similar forms occur in some of the other Romance languages. These forms are evidently made up of the Latin infinitive cantare, depending on habeo ("I have to sing"). But the future in literary Latin was cantabo, formed by adding an ending, as we know, and with that the Romance future can have no connection. However, as a writer in the Archiv has pointed out,[18] just such analytical tense forms as are used in the Romance languages to-day are to be found in the popular Latin sermons of St. Jerome. From these idioms, common to Italian, French, and Spanish, then, we can reconstruct a Latin formation current among the common people. Finally a knowledge of the tendencies and practices of spoken English helps us to identify similar usages when we come upon them in our reading of Latin. When, for instance, the slave in a play of Plautus says: "Do you catch on" (tenes?), "I'll touch the old man for a loan" (tangam senem, etc.), or "I put it over him" (ei os sublevi) we recognize specimens of Latin slang, because all of the metaphors involved are in current use to-day. When one of the freedmen in Petronius remarks: "You ought not to do a good turn to nobody" (neminem nihil boni facere oportet) we see the same use of the double negative to which we are accustomed in illiterate English. The rapid survey which we have just made of the evidence bearing on the subject establishes beyond doubt the existence of a form of speech among the Romans which cannot be identified with literary Latin, but it has been held by some writers that the material for the study of it is scanty. However, an impartial examination of the facts ought not to lead one to this conclusion. On the Latin side the material includes the comedies of Plautus and Terence, and the comic fragments, the familiar odes of Catullus, the satires of Lucilius, Horace, and Seneca, and here and there of Persius and Juvenal, the familiar letters of Cicero, the romance of Petronius and that of Apuleius in part, the Vulgate and some of the Christian fathers, the Journey to Jerusalem of St. AEtheria, the glossaries, some technical books like Vitruvius and the veterinary treatise of Chiron, and the private inscriptions, notably epitaphs, the wall inscriptions of Pompeii, and the leaden tablets found buried in the ground on which illiterate people wrote curses upon their enemies.
It is clear that there has been preserved for the study of colloquial Latin a very large body of material, coming from a great variety of sources and running in point of time from Plautus in the third century B.C. to St. AEtheria in the latter part of the fourth century or later. It includes books by trained writers, like Horace and Petronius, who consciously adopt the Latin of every-day life, and productions by uneducated people, like St. AEtheria and the writers of epitaphs, who have unwittingly used it.
St. Jerome says somewhere of spoken Latin that "it changes constantly as you pass from one district to another, and from one period to another" (et ipsa Latinitas et regionibus cotidie mutatur et tempore). If he had added that it varies with circumstances also, he would have included the three factors which have most to do in influencing the development of any spoken language. We are made aware of the changes which time has brought about in colloquial English when we compare the conversations in Fielding with those in a present-day novel. When a spoken language is judged by the standard of the corresponding literary medium, in some of its aspects it proves to be conservative, in others progressive. It shows its conservative tendency by retaining many words and phrases which have passed out of literary use. The English of the Biglow Papers, when compared with the literary speech of the time, abundantly illustrates this fact. This conservative tendency is especially noticeable in districts remote from literary centres, and those of us who are familiar with the vernacular in Vermont or Maine will recall in it many quaint words and expressions which literature abandoned long ago. In Virginia locutions may be heard which have scarcely been current in literature since Shakespeare's time. Now, literary and colloquial Latin were probably drawn farther apart than the two corresponding forms of speech in English, because Latin writers tried to make the literary tongue as much like Greek in its form as possible, so that literary Latin would naturally have diverged more rapidly and more widely from conversational Latin than formal English has drawn away from colloquial English.