But not only may talking birds attach appropriate significations to nouns, adjectives, and verbs; they may even use short sentences in a way serving to show that they appreciate—not, indeed, their grammatical structure—but their applicability as a whole to particular circumstances.[84] But this again is not a matter to excite surprise. For all such instances of the apposite use of words or phrases by talking birds are found on inquiry to be due, as antecedently we should expect that they must, to the principle of association. The bird hears a proper name applied to a person, and so, on learning to say the name, henceforth associates it with that person. And similarly with phrases. These with talking birds are mere vocal gestures, which in themselves present but little more psychological significance than muscular gestures. The verbal petition, “Scratch poor poll,” does not in itself display any further psychological development than the significant gesture already alluded to of depressing the head against the bars of the cage; and similarly with all cases of the appropriate use of longer phrases. Thus, supposing it to be due to association alone, a verbal sign of any kind is not much more remarkable, or indicative of intelligence, than is a gesture sign, or a vocal sign of any other kind. The only respect in which it differs from such other signs is in the fact that it is wholly arbitrary or conventional; and although, as I have previously said, I do consider this an important point of difference, I am not at all surprised that even the intelligence of a bird admits of such special associations being formed, or that a wholly arbitrary sign of any kind should here be acquired by this means, and afterwards used as a sign.