The nearest approach to language are the sounds uttered by birds. All that sing exert their power instinctively, but the actual song, and even the call notes, are learned from their parents or foster-parents. These sounds are no more innate than language is in man, as has been proved by Davies Barrington.[58] The first attempt to sing "may be compared to the imperfect endeavor in a child to babble." Prof. Whitney says, if the last transition forms of man "could be restored, we should find the transition forms toward our speech to be, not at all a minor provision of natural articulate signs, but an inferior system of conventional signs, in tone, gesture, and grimace. As between these three natural means of expression, it is simply by a kind of process of natural selection and survival of the fittest that the voice has gained the upper hand, and come to be so much the most prominent that we give the name of language (tonguiness) to all expression." A single utterance or two at first had to do the duty of a whole clause; afterward man learned to piece together parts of speech, and thus arose sentences. Although no language, as has already been said, has been deliberately invented, "still each word may not be unfitly compared to an invention; it has its own place, mode, and circumstances of devisal, its preparation in the previous habits of speech, its influence in determining the after progress of speech development; but every language in the gross is an institution, on which scores or hundreds of generations and unnumbered thousands of individual workers have labored."[59] There is no question at all but that the mental powers in the earliest progenitors of man must have been more highly developed than in the ape, before even the most imperfect form of speech could have come into use; but the constant advancement of this power would have reacted on the mind to enable it to carry on longer trains of thought. "A complex train of thought," says Darwin, "can no more be carried on without the aid of words, whether spoken or silent, than a long calculation without the use of figures in algebra. It appears also that even an ordinary train of thought almost requires or is greatly facilitated by some form of language; for the dumb, deaf, and blind girl, Laura Bridgman, was observed to use her fingers while dreaming.[60] Nevertheless a long succession of vivid ideas may pass through the mind, without the aid of any form of language, as we may infer from the movements of dogs during their dreams."