That which proves besides that the barking of the dog is merely a conventional sign, an _artificial language_, so to say, is the fact, that in certain countries the dogs do not bark; jackals and wolves learn how to bark when in company with the dogs who can talk in this manner, and that the same dogs lose the power, or rather the habit of barking, if they return to a savage state.[90] We have already spoken of those inferior races which seem to have borrowed from their better endowed neighbours a rudiment of civilisation, which, for a long time they did not know how to develope in any way. Does it not seem that there is here some comparison with what has just been stated? that under a civilising influence, in contact with a superior being, the dog has learnt a language; but that not understanding its general application (a more complex, and more highly elevated idea), he has not known how to transmit the use of it to his own race, or has himself forgotten it, from not having any occasion to exercise this power?[91] The language of animals is still a question full of obscurity, but which may eventually, we believe, become fruitful in new facts.[92] If Apollonius of Tyana and the ancient philosophers did not understand the language of animals so well as has sometimes been believed, at least they did not do wrong in directing their inquiries towards this matter. We have no doubt but that in carefully studying animals, we shall arrive at a scientific explanation of this well-known truth, recognised by all those who live with them; which is that they can understand us, that they make themselves understood by us, and that they understood one another within certain limits.[93] For a long time it was believed that intellect and thought belong to man alone, and that he had only organic instinct in common with animals.[94] This opinion tends each day towards a change; we hope that we have proved so much. Something of the same sort will take place, we think, for the best studied language. There, as with intellect, as with organism, we shall doubtless be able to prove a unity which may be regarded by analogy as necessary, offering alone degrees of gradation in reference to organism and intellect. Every living being (we are only speaking here of vertebrate animals) will appear to be composed of the same constituent parts, but unequally developed, and of which some have only been taken by us for dissimilarities or new parts, on account of our own want of sufficiently deep study. As they formerly tried to discover new bones in the heads of fishes, until the time when their relations, connexions, and development were better studied; so unity of composition has been there recognised and proved where it was least suspected.