The brains of man and ape being fundamentally the same in structure, it follows that the functions which they perform are fundamentally the same. The large array of facts mustered by a series of careful observers prove how futile is the argument which, in his pride of birth, man advances against psychical continuity. Vain is the search after boundary lines between reflex action and instinct, and between instinct and reason. Barriers there are between man and brute, for articulate speech and the consequent power to transmit experiences has set up these, and they remain impassable. "The potentialities of language, as the vocal symbol of thought, lay in the faculty of modulating and articulating the voice. The potentialities of writing, as the visual symbol of thought, lay in the hand that could draw, and in the mimetic tendency which we know was gratified by drawing as far back as the days of Quaternary man" (Huxley's Essays on Controverted Questions, p. 47). But these specially human characteristics are no sufficing warrant for denying that the sensations, emotions, thoughts, and volitions of man vary in kind from those of the lower creation. "The essential resemblances in all points of structure and function, so far as they can be studied, between the nervous system of man and that of the dog, leave no reasonable doubt that the processes which go on in the one are just like those which take place in the other. In the dog, there can be no doubt that the nervous matter which lies between the retina and the muscles undergoes a series of changes, precisely analogous to those which, in the man, give rise to sensation, a train of thought, and volition." This passage occurs in Huxley's Reply to Mr. Darwin's Critics, which appeared in the Contemporary Review, 1871, and it may be supplemented by a quotation from the chapter on The Mental Phenomena of Animals in his Hume. "It seems hard to assign any good reason for denying to the higher animals any mental state or process in which the employment of the vocal or visual symbols of which language is composed is not involved; and comparative psychology confirms the position in relation to the rest of the animal world assigned to man by comparative anatomy. As comparative anatomy is easily able to show that, physically, man is but the last term of a long series of forms, which lead, by slow gradations, from the highest mammal to the almost formless speck of living protoplasm, which lies on the shadowy boundary between animal and vegetable life; so, comparative psychology, though but a young science, and far short of her elder sister's growth, points to the same conclusion."