INTRODUCTION 



fertilising purposes. This accomplished, no further 

 heed is paid to what happens, and fishes seem to resemble 

 insects in this respect. Exception must be made, how- 

 ever, in the case of that plucky little " soldier," the 

 Stickleback, who not only builds a nest, but jealously 

 guards it and his wife against invasion. It is a notable 

 exception of great interest, and will be referred to later 

 when the life-story of this favourite species is being 

 unfolded. 



Of the external appearance of a fish it need only be said 

 here that, whilst some have large scales — like the Roach 

 and Rudd — others bear very small ones — like the bottom- 

 loving Gudgeon — and others again — like the Eel and 

 Tench — are practically scaleless as compared with their 

 heavily-plated brethren. The tapering body, that is, 

 having the head and hind quarters narrower and more 

 torpedo-shaped than the deeper central parts, is well 

 known, and its use in permitting the owner to propel 

 itself through the water with the least resistance is 

 equally familiar. Not that all our fishes are thus shaped, 

 for the Eel and Lamprey are snake-like in appearance, 

 and certain marine species are far different in form, 

 but the typical example will serve our purpose. The 

 body bears upon it fins equivalent to the limbs possessed 

 by a quadruped, or the wings and legs of a bird. If 

 we take that voracious and social fresh-water inhabitant, 

 the Perch, as a basis for explanation, we find that along 

 the back it has two fins, the one in front being called the 

 first dorsal, and the one immediately behind is the second 



