182 THE STORY OF FISH LIFE. 



are highly specialised in that the anterior end 

 of the head is produced forward into a broad 

 shovel-like process, used, it is supposed, as an 

 organ of touch : and degenerate in that, the 

 eyes are so extremely small as to render them of 

 but little use. This degeneration of the eyes, 

 and the compensatory organ of touch, seems to 

 have been induced by the turbidity of the rivers 

 in which they lived, which is so great as to 

 render eyes almost useless. Another degenerate 

 feature is seen in the scales, which are very 

 minute and star-shaped. Teeth are retained 

 throughout life. From the living Polyodon we 

 are carried geologically back to the fossil genus 

 Chondrosteus of the Lias formation, which, like 

 Polyodon, had, amongst other things, developed a 

 tactile paddle. 



In studying the animal life of the globe, from 

 its earliest dawn till to-day, we shall find evidence 

 of a gradually increasing complexity therein. 

 Furthermore, if we select any particular group 

 of animals for review, we shall not fail to be 

 struck with the fact that that group exhibits a 

 series of characteristic forms, the rising, waxing, 

 and waning of which may extend through one 

 or more geological periods, and then suddenly 

 die out ; or it may persist under greatly modified 

 aspects till to-day, either in the form of an isolated 

 survivor of an ancient race, or as a congeries of 

 forms in the hey-day of development. 



In the history of the house from which the 

 sturgeons derive their origin, we may find some 

 instructive instances of this rise and decline. 

 The earliest representatives of this house, we 



