Now it is very natural that many such fish should learn to remain in this quiet refuge, and in time to live there altogether. And because fish-life in the rivers is comparatively uneventful and little varied, we find much fewer peculiarities in river-fish. Many of them are very near relations of sea forms. There is the salmon, a true sea-fish, which wanders up the river to spawn in the pebbly shallows; and there are the trout, his near relations, which have learned to live entirely in the rivers. There are the sea-perches, large strong fish, and the smaller river perch, which have made their homes very successfully in the rivers, for their spines are so sharp that even the greedy pike hesitates to swallow them. There are the sea-sticklebacks, and the little river-stickleback.[50] This last is a very clever little fish, which hollows out the foundation of his nest very carefully in the bed of the river, and then builds it up for several inches with blades of grass and weeds (Fig. 14), gumming them together with the slime of his body. Then, when all is ready, he swims about to drive and coax the mother to the nest, sending her in to lay her eggs, and then driving her right through and out at the other side, so that a stream of water flows constantly over the eggs till they are hatched. Nay, his care does not end here, for when the young fish come out of the egg with a bag of yelk hanging under the body, as all young fish have at first, and so cannot swim easily and escape their enemies, the courageous little father will defend them and fight fiercely with any fish which thinks to make a meal upon them, not leaving them till all the yelk is absorbed, and they are able to swim and feed themselves. [Illustration: Fig. 14. STICKLEBACKS AND THEIR NEST. (Gasterosteus aculeatus.)]