INTRODUCTION. 15 



the highly specialised floating one. The seal is 

 amphibious, but the hind-limbs are already los- 

 ing much of their power of support on land, and 

 the fore-limbs are becoming more and more 

 paddle-shaped. 



The old ichthyosaurus is but one of many 

 forms amongst the extinct reptiles which have 

 undergone a precisely similar modification to 

 that of the whale in the shape of the body and 

 limbs. Certain living snakes again, by adopting 

 an entirely aquatic existence, have become quite 

 eel-like in form, rendering it very difficult to 

 distinguish eel from snake. But there is no 

 need to go on multiplying instances of this kind. 

 The facts are beyond dispute. The fish, there 

 can be no doubt, owes its peculiar form to the 

 gradual adaptation to the needs of its environ- 

 ment. 



Fishes hold, says Dr Bashford Dean, an im- 

 portant place in the history of vertebrate or 

 backboned animals ; their group is the largest 

 and most widely distributed ; its fossil members 

 are by far the earliest of known vertebrates ; and 

 amongst its living representatives are forms 

 which are believed to closely resemble the 

 ancestral vertebrate. 



The origin of new groups of fishes yet remains 

 a mystery, but certain facts connected therewith 

 afford us food for reflection of extraordinary 

 interest. These facts have lately been set forth 

 with telling force by Dr A. Smith Woodward. 

 He points out that in tracing the history of the 

 evolution of any given group of animals, say of 

 fishes, we find that during different geological 



