That animals of the higher grades, at least, have this knowledge is very evident; gregarious habits, mutual recognition, the watch and ward system, characteristic of so many species both among mammalia and birds--a thousand circumstances which our readers may call to mind, prove it. We need not insist upon the subject. It is a feeling rather than a portion of knowledge; it is intuitive, and experienced alike by the ploughman at his work, by the horses who labour in the furrow, and the busy rooks which follow in their track. Where there is fear, or pleasure, or pain, or desire, there must self-consciousness be--a feeling of self-existence. We have no reason to believe that plants possess this feeling. 2. _Memory._--Memory is a mental exercise, or faculty, of the greatest importance to man; it is in constant requisition; without it our intellectual improvement would be confined within very narrow limits. Our acquirement of language, and of the rules of sciences and arts, depend upon this faculty, and it is strengthened by habitual exercise. Between memory and recollection, there is a slight shade of difference. A man may say, "I remember the events of the year 1848"--"I recollect such an event, now that you mention it." Events may be recollected after being forgotten, and often are so in illness--nay, forgotten languages have sometimes been fluently spoken. It often happens--why so, we cannot say--that circumstances or events, long passed from the mind, present themselves suddenly to it with extreme vividness, and are thenceforth remembered; but oftener does it happen, that the sight of some object, or that a sound, a note, a tune, a word, recalls to the mind forgotten events, bygone circumstances, hopes passed away, pleasures long fled, feelings long quiescent. This species of recollection has been called by some writers the "power of association." It certainly depends upon an association of ideas, which, on the key-note being struck, pass involuntarily through the mind; a sight or sound will often call to mind, with singular rapidity, a vision of other years, or of scenes far distant, towards which the heart intently yearns. Witness the effects of the air of the Ranz de Vache upon the Swiss soldiers in Napoleon's army. Recollection is often the result of attention. We remember a certain event, but forget the year in which it occurred; we direct our thoughts to the subject, we compare circumstance with circumstance, date with date, until at last-- "It breaks upon the mind, and all is clear."