STRUCTURE AND MECHANISM OF FISHES 17 



ing it upright, how some forms are provided with powerful searchlights 

 for illuminating the dark depths in which they live. But, verbum sat 

 sapienti, let us investigate these things for ourselves. 



Turning now to a consideration of fishes as living creatures rather 

 than as machines it is interesting to note that sharks, which have a very 

 simple body structure with practically no improvement since an early 

 period in the earth's history, have reached a very high plane in the evolu- 

 tion of reproduction. While modern fishes usually lay a great many 

 small eggs which are fertilized by chance in the water, this casual method 

 of reproduction means much wastage, as many of the eggs are lost or 

 consumed by predaceous animals, and the young, when hatched, are 

 small and comparatively helpless. In most sharks, on the other hand, 

 the eggs hatch and the young reach an advanced stage of development 

 within the body cavity of the mother, and in some there are complicated 

 mechanisms for their nourishment before birth, comparable with the 

 condition found in mammals. In the few species which still lay eggs, 

 these are large and protected by a leathery egg-case. It would seem that 

 by specialization in the one matter of reproduction, and the consequent 

 elimination of a great deal of waste, the sharks living at the present time 

 are able to compete successfully with other fishes, which are more modern 

 and better adapted to their environment in many other ways. 



