408 t. C CHAMBERLIN 



natural mode of neutralizing the current force and securing 

 stability of position or motion against the current, as desired. 

 Beyond question the form and the movement of the typical fish 

 are admirably adapted to motion in static water and that has 

 been thought a sufficient reason for the evolution of the form, and 

 so possibly it may be, but fishes in static water have not as 

 uniformly retained the attenuated spindle-like form and the 

 extreme lateral flexibility as have those of running water. 

 Among these latter it is rare that any great departure from the 

 typical "lines" and from ample flexibility has taken place, while 

 it is not uncommon in sea fishes. Among the latter not a few 

 have lost both the typical form and the flexibility. The porcu- 

 pine-fish, the sea-horse, the flounders, and many others are 

 examples of such retrogressive evolution, which is doubtless 

 advantageous to them within their special spheres in quiet waters,. 

 but would quite unfit them for life in a swift stream. And if 

 the view be extended to include the low degenerate forms, like 

 the Ascidians, that are by some authors classed as chordates, 

 the statement finds further emphasis. 



It is not difficult for the imagination to picture a lowly 

 aggregate of animal cells, still plastic and indeterminate in 

 organization, brought under the influence of a persistent current 

 and caused to develop into 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 interfer- 

 ing with lateral flexibility, is obvious and is supplied by a noto- 

 chord. Thus,^,by hypothesis, the primitive chordate form may 

 be regarded as a specific response to the special environment 

 that dominated the evolution of a previously indeterminate 

 ancestral form. 



