The adverbial meaning of so many of the cases explains the readiness with which they became crystallized into adverbs and prepositions. An adverb is the attribute of an attribute--"the rose smells sweetly," for example, being resolvable into "the rose has the attribute of scent with the further attribute of sweetness." In our own language _once_, _twice_, _needs_, are all genitives; _seldom_ is a dative. The Latin and Greek _humi_ and [Greek: chamai] are locatives, _facillime_ (_facillumed_) and [Greek: eutychôs] ablatives, [Greek: pantê] and [Greek: hama] instrumentals, [Greek: paros], [Greek: hexês] and [Greek: têlou] genitives. The frequency with which particular cases of particular nouns were used in a specifically attributive sense caused them to become, as it were, petrified, the other cases of the nouns in question passing out of use, and the original force of those that were retained being gradually forgotten. Prepositions are adverbs employed to define nouns instead of verbs and adjectives. Their appearance in the Indo-European languages is comparatively late, and the Homeric poems allow us to trace their growth in Greek. The adverb, originally intended to define the verb, came to be construed with the noun, and the government of the case with which it was construed was accordingly transferred from the verb to the noun. Thus when we read in the _Odyssey_(iv. 43), [Greek: autous d eisêgon theion domon], we see that [Greek: eis] is still an adverb, and that the accusative is governed by the verb; it is quite otherwise, however, with a line like [Greek: Atreidês de gerontas aolleas êgen Achaiôn es klisiên] (Il. i. 89) where the adverb has passed into a preposition. The same process of transformation is still going on in English, where we can say indifferently, "What are you looking at?" using "at" as an adverb, and governing the pronoun by the verb, and "At what are you looking?" where "at" has become a preposition. With the growth and increase of prepositions the need of the case-endings diminished, and in some languages the latter disappeared altogether.
Like prepositions, conjunctions also are primarily adverbs used in a demonstrative and relative sense. Hence most of the conjunctions are petrified cases of pronouns. The relation between two sentences was originally expressed by simply setting them side by side, afterwards by employing a demonstrative at the beginning of the second clause to refer to the whole preceding one. The relative pronoun can be shown to have been in the first instance a demonstrative; indeed, we can still use _that_ in English in a relative sense. Since the demonstrative at the beginning of the second clause represented the first clause, and was consequently an attribute of the second, it had to stand in some case, and this case became a conjunction. How closely allied the adverb and the conjunction are may be seen from Greek and Latin, where [Greek: hôs] or _quum_ can be used as either the one or the other. Our own _and_, it may be observed, has probably the same root as the Greek locative adverb [Greek: eti], and originally signified "going further."