vii FINS AND LIMBS 441 



They clearly represent successive grades in the evolution of the tail as 

 a more and more perfect organ of propulsion, a process of evolution 

 which has come about independently in the various groups of fishes. 

 Thus even the Lung-fishes — the surviving members of which group 

 possess the primitive protocercal type of tail — during the geological 

 periods when they most flourished showed numerous forms in which 

 there was a highly developed heterocercal tail. 



Again the assumption of a sluggish mode of life, or the simpli- 

 fication of the swimming movements, is frequently correlated with 

 reversion of the tail towards the protocercal condition. This is 

 clearly seen in many teleostean fishes, such as the Eels and many 

 deep-sea bottom-frequenting fishes. In such cases all trace of the 

 unsymmetrical condition may have disappeared from ontogeny but 

 there is no room for doubt regarding an ancestral heterocercal or 

 homocercal condition — for in their general structure these fishes are 

 highly evolved Teleosts and the group as a whole is characterized by 

 the tail being homocercal. 



In the case of the surviving Lung-fishes the general archaicism 

 in structure, and more especially the extremely archaic character of 

 the paired fins, are in favour of the protocercal character of the tail 

 being persistent rather than revertive — apart from the evidence of 

 embryology which fails to disclose any trace of a pre-existing hetero- 

 cercal phase. 



(3) The Limbs. — One of the characteristic structural features oi 

 the Vertebrata is the presence of the two pairs of limbs, pectoral and 

 pelvic. Two main types of such limbs can be recognized — the fin 

 type for swimming and the pentadactyle or leg type for moving on a 

 solid substratum. As the former is on the whole characteristic of 

 fishes, and as fishes are on the whole more nearly primitive than are 

 terrestrial Vertebrates, the idea has naturally arisen and has now 

 attained perilously near to the position of a dogma that the leg type 

 of hmb has been evolved out of the fin ; and elaborate attempts have 

 been made to define the manner in which this has come about. It 

 is necessary at the outset to emphasize the importance of keeping an 

 open mind upon this question : there exists the possibility — which as 

 will be seen later is not lightly to be brushed aside — that penta- 

 dactyle limb and fin are not in the relation of lineal descent at all 

 but that they have been derived from a common ancestral type of 

 limb differing from either. 



No limbs exist in Amphioxus or in the Cyclostomata. There is 

 however a general tendency in Vertebrates which have assumed an 

 eel-like form of body for the limbs to degenerate and disappear, and 

 it is well to bear in mind the possibility that this has happened in 

 the case of both of the types mentioned. 



The limb at its first appearance in embryonic development forms 

 a little projection from the body surface — a core of mesenchyme 

 enclosed in an ectodermal sheath. In Lepidosiren or Protopterus or 

 a Urodele it is in the form of a rounded knob identical in appearance 



