By and by the boys noticed that every time one of the little fish darted at another, the three cruel spines rose up on his back, and that he was really trying to spear his neighbour. One morning a dead stickle-back was found in the bottom of the tank. A few days later another little fish was picked out pale and stiff. “They are killing one another,” said Frank. “What shall we do?” “If any more of this fighting goes on we shall have to put them back into the brook,” said Uncle George. “Do they always fight?” “No, not always—only in spring-time when they are mating. Look! there is one of them getting very pretty. He is the victor—the bully of the pool.” “Let us call him _Bully_,” said Dolly; “he is bigger than the others, and oh, so much more beautiful.” Next day another stickle-back was found dead, and Bully’s colours were much brighter. He darted about as if the whole tank belonged to him. He was really a lovely fish now, and he seemed to know it by the proud way in which he dashed about, showing off his fine slender body all shiny with crimson, blue, and gold. He was, as Dolly said, “Just like a little bit of rainbow.” But before the evening a very curious thing took place. Bully seemed to have suddenly lost all his fine colours; and instead of swimming proudly at the top of the tank, he slunk sulky to the bottom. The strange thing was that _another_ stickle-back—a smaller fish than Bully—was now brightly coloured, and seemed to be lord of the tank. “Bully has been beaten,” said Uncle George, “and his victor has taken not only the courage but the colour out of him.” “It serves him right, I think, for being so proud and so cruel. But what is the meaning of all this fighting and change of colours, Uncle George?” “Oh, it is very simple, Frank. There is a lady stickle-back in the question; and, like the brave knights of old, our little stickle-backs are trying to win her by fighting. “The victor will marry her. They will build a neat little nest for themselves, and live happily together. “To-morrow we will take them back to the brook, where the weak ones will be better able to escape. “In June we will visit the brook. If we are lucky enough to find one of their nests, you will see that after Lady Stickle-back lays her tiny eggs in it, her little husband guards the home night and day. “When the family are hatching out, the plucky little stickle-back bravely defends the nest. “He drives away water-beetles, perch, and other fishes much larger than himself. For well he knows that these visitors would quickly gobble up his darlings if they got the chance.” [Illustration: Stickle-backs and Nest.] Exercises on Lesson VIII. 1. What is an aquarium? 2. Why are water weeds and water snails put into an aquarium? 3. How do you feed small fish? Why should you be careful not to put in more than the fishes can eat? 4. Where does the stickle-back lay its eggs?