AND ITS INHABITANTS 117 



in the horizontal plane, as that is the direction of least re- 

 sistance because of the inhibitive force of gravity. When the 

 lamprey wishes to swim to maintain its position in the current, 

 it simply reproduces in the active voice the motions imparted 

 to it in the passive voice by the stream itself. More forcible 

 action drives it ahead. Such motion, consisting as it does of 

 a series of reversed curves, requires segmented muscles on 

 either side, along which alternating waves of contraction may 

 pass. An axial stiffening to resist the longitudinal compres- 

 sion of such muscles is also necessary. This is first a pliant 

 cellular membrane rendered resistant by being tensely filled 

 with liquid, next a supple rod of cartilage, and finally a bony 

 axis, segmented for the sake of flexibility. The economic effi- 

 ciency of this undulatory mode of progression as compared 

 with the jet propulsive method of the cephalopod is evident to 

 anyone who has observed the quick dart of a trout or the 

 marvelously effortless progress of a school of porpoises at the 

 bow of a ship. 



As time went on, the assumption of the compact spindle 

 shape of a swiftly swimming fish became, as we have argued, 

 merely a speed response and not necessarily an exclusively 

 stream-borne attribute. Depressed grovelling forms (see Fig. 

 1 6, C), or compressed highly specialized forms, are, however, 

 either marine or static fresh-water in habitat, never stream 

 dwellers. 



It has been argued that as the most primitive existing 

 chordates, Amphioxus, the tunicates, and the adelochordates 

 (Balanoglossus}, are marine, the ancestral chordates must 

 have been also, but their distribution corroborates the hy- 

 pothesis of fresh-water origin instead of denying it, for as 

 Matthew 2 has shown, the most ancient members of a group 

 are not to be found at the old center of evolution, bijt rather 



2 Matthew, W. D., Climate and evolution. Ann. New York Acad. Sci., vol. 

 24, 1915, p. 180. 



