Most people will only grant a moderate degree of intelligence to fish, linking coldness of blood to narrowness of intellect, and convinced that there can be but little brain in so small a compass as its head. That the jack can compete with the dog, of course, is out of the question: but I am by no means prepared to admit that fish are so devoid of sense as supposed. Not long since an experiment was tried with a jack, an account of which appeared in the papers. The jack was in a tank, and after awhile the tank was partly divided by inserting a plate of glass. He was then hunted round, and notes taken of the number of times he bumped his head against the plate of glass, and how long it took him to learn that there was something to obstruct his path. Further statistics were kept as to the length of his memory when he had learnt the existence of the glass--that is, to see if he would recollect it several days afterwards. The fish was some time learning the position of the glass; and then, if much alarmed, he would forget its position and dash against it. But he did learn it, and retained his memory some while. It seems to me that this was a very hard and unfair test. The jack had to acquire the idea of something transparent, and yet hard as wood. A moment's thought will show how exactly opposite the qualities of glass are to anything either this particular fish or his ancestors could have met with--no hereditary intelligence to aid him, no experience bearing, however slightly, upon the subject. Accustomed all his life to transparent water, he had also been accustomed to find it liquid, and easily parted. Put suddenly face to face with the transparent material which repelled him, what was he to think? Much the same effect would be produced if you or I, having been accustomed, of course, all our lives, to the fluidity of air, which opens for our passage, were opposed by a solid block of transparent atmosphere. Imagine any one running for a train, and striking his head with all his might against such a block. He would rise, shake himself together, and endeavour to pursue his journey, and be again repelled. More than likely he would try three times before he became convinced that it really was something in the air itself which stopped him. Then he would thrust with his stick and feel, more and more astounded every moment, and scarcely able to believe his own senses. During the day, otherwise engaged, he would argue himself into the view that he had made a mistake, and determine to try again, though more cautiously. But so strong is habit that if a cause for alarm arose, and he started running, he might quite probably go with tremendous force up to the solid block of transparent air, to be hurled back as the jack was.