MORPHOLOGICAL IMPORTANCE. 45 



many other points, Ampliioxus remains throughout life in a 

 condition characteristic of the early developmental phases of the 

 higher Vertebrates. Amphioxus halts permanently at a st age 

 th rough which all the higher Vertebrates pass durin g their 

 d evelopment. 



The Recapitulation Theory explains th is as indicating that 

 in these r^pyuLy Ailiphiuj^us i'epiBsents, more or less exactly, a 

 phase through which the higher Vertebrates have passed in the 

 history of their evolution ; that, as regards the organs in ques- 

 tion, Amphioxus may be viewed as figuring, with more or less 

 exactness, an ancestral form from which the higher types of 

 Vertebrates are descended. 



From this standpoint Amphioxus is an animal of verj- 

 special importance to morphologists ; and the development of 

 Amphioxus acquires peculiar interest from the consideration 

 that, if the adult animal is far more primitive than any other 

 existing Vertebrate, t hen the earlier stages in its life history may 

 re asonablv be expected, in accordance with the law of Rec api- 

 tulat ion, to vield valuable py'dpHP.)^ as tn th e relations of Verte^ 

 brates with th e n imp^"" g^-'^-'-'p" "-^ "^^°^-i i 7 i nfl 



The above considerations do not imply that Amphioxus 

 itself stands in the direct line of ancestry of any of the higher 

 Vertebrates, but that it is a surviving representative of a type 

 of animals which preceded the higher Vertebrates in point of 

 time, and from which type, though not necessarily from Amphi- 

 oxus itself, the higher Vertebrates have arisen. 



Amphioxus shows us that, in attempti ng to reconstru ct the 

 characters of th e ancestors of Vertebr ates, we are almost certain ly 

 ■jn stifip.fl in omitting such features as paired lim bs, a jjartila - 

 mnnng rvr Vi nj ^ y sirq l ptnn . jaws, a tw jsted or chambered heart, a 

 highly special ise'^ b^m 'n and paire d sense o rgans ; characters 

 which Amphioxus shows us are not necessary to an adult 

 Vertebrate, and in the absence of which the embryos of higher 

 Vertebrates agree with Amphioxus. 



A diflferent explanation of the peculiarities of Amphioxus has 

 been offered by many zoologists, who consider that the simplicity 

 that characterises so many of its organs, as the brain, heart, 

 liver, &c., is not primitive, but due to degeneration ; that the 

 immediate ancestors of Amphioxus were, in fact, animals higher 

 in the zoological scale than itself. No distinct evidence of suck 



