10 SATURDAY LECTURES. 



certain forms that are in some measure intermediate be- 

 tween the cetaceous and ordinary quadrupeds ; that is, in 

 extinct forms known under the name of Zeuglodonts which 

 have not existed since the eocene epocli, and in ancient 

 forms of the group, (Sirenians,) to which tli(3 Manatee, and 

 the Dugong of the present time belong. 



Reverting now to the ventral fins of the fishes, 3^ou will 

 perceive that they correspond in position with the hind legs 

 of tadpoles. If we compare the ventral fins of certain fishes 

 of a very generalized kind, as it is called — that is, like 

 those forms from which other fishes and amphibians are 

 supposed to have alike descended — with certain extinct 

 amphibians, also of a generalized kind we are prepared to 

 recognize a similarity, and ultimately an equivalency of 

 those ventral fins with the hind feet of quadrupeds. (The 

 equivalency, let me state here, is called homology.) The 

 so-called generalized fish which is most instructive in its 

 indications is the Polypterus, (there is no common English 

 name,) of the Nile, and other African rivers. The general- 

 ized amphibians are known as Lab^a-inthodonts, etc. 



You will, I think, have no hesitation in recognizing the 

 expanded vertical membrane of the tail of the tadpole as 

 equivalent, so far as use is concerned, at least, with the tail or 

 caudal fin of the fish. Let me further direct your atten- 

 tion to the fact, that the fish's tail and the tadpole's tail is 

 median as well as vertical. If you are now prepared to 

 concede all the propositions enunciated, you will be pre- 

 pared likewise to give assent to the proposition, that the 

 whale's flukes in horizontal position, and in being paired, 

 are more like the seal's flippers, and therefore comparable 

 with the hind limbs ; and therefore unlike the tail of a fish. 

 Inasmuch, further, as the seal's flippers are modified hind 

 legs, and the ventral fins are also representatives of the hind 

 legs, the whale's flukes are also modified hind limbs, and 

 therefore the flukes and the ventral fins are equivalent, in 

 a homological sense, to each other. 



But how about the fore fins or pectoral feet ? Are not 

 thev much alike in whales and true fishes? 



