BRITISH FRESH-WATER FISHES
The illustrations accompanying the history of the Trout (Figs. 33 to 36) will convey a better impression than any written description of the life-cycle of a fish, and it may be taken as a typical example of many, if not most, of them. Reference should be made to the above illustrations, and also to the notes upon the Trout on page 67.
Fishes obtain oxygen from the air dissolved in the water, by means of internal gills, but it should be pointed out that all creatures that live in water, although vertebrates, like fishes, do not belong to the finny tribe. The Whale and Seal need only be mentioned as examples of these, both, of course, being mammals and not fishes.
For lessons on the geographical distribution of fishes the reader must be referred to a larger treatise than this popular little volume pretends to be, but a few words must be written, by way of concluding this introduction, as to what constitutes a fresh-water species. It does not follow that because the familiar Stickleback is found in a wayside streamlet, so tiny that one can leap across it, that it does not also inhabit the sea. The reverse is also true, namely, it does not follow that because the Shark is found in the sea that it also occurs in fresh water. The fact is some kinds of fishes are equally well at home in fresh or salt water, and the same species is also found occupying both territories. Then again there are exclusive fresh- water dwellers, as there are exclusive sea- water dwellers, and there are again others, like the Salmon, which spawns in fresh water, and others,
