io8 



Cll 'AFTERS ON EVOLUTION. 



of man. The laws of life, like those of matter, indeed, are absolutely 

 inflexible throughout ; and the story of a fish-tail and its develop- 

 ment finds the closest parallel in that chronicle through which evolution 

 traces the production and growth of the entire scheme of nature. 



From the nature and development of the tails of fishes and of 

 other animals we may pass, by an easy transition, to the subject of 

 limbs, and their modifications. In this latter study we may perchance 

 discover facts and inferences of no less interest than those evolved in 

 our investigation into the history of fishes' tails. The limbs of 

 animals appear before us as out-jutting portions or special out- 

 growths of the trunk or body proper. That there are limbs and 

 limbs is a very evident fact to any one who considers the wide varia- 

 tions which exist between the similarly named parts in an insect or 

 centipede, a fish, a bird, a whale, a dog, and a man. And even 

 within the limited compass of our own frames, there would appear at 

 first sight to be an essential difference betwixt the arm and leg, and 

 an equally great distinction between the fore-limbs, or " wings," of a 

 bird or bat, and the hind limbs of these animals. A fish, too, might 



FIG. 47. FORE-LIMBS OF VARIOUS VERTEBRATE ANIMALS. 

 A, man ; B, horse ; c, bird ; D, whale ; E, fish ; F, bat. 



popularly be supposed to want limbs ; but, as the sequel will show, 

 most fishes possess very distinct representatives of the bodily ap- 

 pendages seen in higher animals, and associated with the movements 

 of the frame. Leaving the limbs of invertebrate animals out of sight 

 for the nonce, we may find that, despite the apparent dissimilarity of 

 form and functions, the limbs of vertebrates present an identity of 



