THE ORIGIN AND EVOLUTION OP THE VERTEBRATES 75 



and caused to develop into a determinate organization under its 

 control, and hence to acquire, as its essential features, a spindle- 

 like form, a lateral flexibility, and a set of longitudinal side muscles 

 adapted to rhythmical contractions, since these are but expressions 

 of conformity and responsiveness to the shape and movement 

 normally impressed by the controlling environment upon plastic 

 bodies immersed in it. The necessity for a stiffened axial tract 

 to resist the longitudinal contractions of the side muscles and 

 thus to prevent shortening without seriously interfering with 

 lateral flexibility, is obvious, and is supplied by the notochord. 

 Thus by hypothesis, the primitive chordate form may be re- 

 garded as a specific response to the special environment that 

 dominated the evolution of a previously indeterminate ancestral 

 form." 

 It will be seen that the Amphioxus theory of vertebrate descent 

 carries with it certain implications: 



(1) That the earliest pro-fishes were of a grade of organization not 

 very different from Amphioxus. 



(2) That the first fishes developed directly from these pro-fishes 

 under the influence of a rapid stream environment. 



(3) That the ostracoderms, the oldest fossil fishes found in the mid- 

 dle Ordovician, are specialized bottom-feeding types and are not an- 

 cestral to any of the modern types. 



(4) That the true ancestral fishes were, according to Osborn, "ac- 

 tive, free-swimming, double-pointed types of fusiform shape, adapted 

 to rapid motion through the water and to predaceous habits in pur- 

 suit of swift-moving prey." 



(5) These primitive fishes must have existed before the Ordovician, 

 i. e., in Cambrian times, which is the earUest period of which we have 

 positive fossil records. This makes the chordates as old as any of the 

 invertebrate phyla and tends to detract from the vaUdity of any 

 theory involving the idea that the chordates have been derived from 

 any of the higher invertebrate phyla. 



(6) That the tunicates, instead of being ancestral to Amphioxus, 

 have been derived from an early Amphioxus-like stock. 



Other Theories of Vertebrate Ancestry 



The traditional method of phylogenetic research has led to the be- 

 lief that each higher group of animals has been derived from one of 



