n6 EVOLUTION OF THE EARTH 



powers, and the evolution of so marked an attribute as high 

 speed must have some powerful incentive. The thesis has been 

 advanced therefore that the main distinction between verte- 

 brate or more properly chordate and invertebrate is largely 

 a response to habitat, the invertebrate being the product of 

 static waters, marine or sluggish terrestrial, where an effort- 

 less existence, carried hither and yon by wave and tide, would 

 not remove it from the environment. The chordate, on the 

 other hand, is the outcome of dynamic or flowing terrestrial 

 waters which enforced swimming powers as the only means of 

 resisting eviction from the realm. The distinction is not one 

 of contour, for the fusiform shape of the speedy animal is the 

 result of swift movement through a more or less resistant 

 medium, whatever the motive power; but the segmental body 

 muscles, the internal resistant axis, and the fin-like expansions 

 to resist the thrust of the muscles, all are the direct outcome of 

 the mode of locomotion, that by lateral undulation of a primi- 

 tively elongated body. (See Fig. 16.) That this locomotive 

 device might have been developed in static waters is not denied, 

 for some marine worm-like organisms swim by a wriggling 

 movement, though invariably in the dorso-ventral plane, but 

 there is in the sea little incentive to enforce its rapid perfec- 

 tion, such as dynamic waters would produce. Hence the 

 assumption that the vertebcale^-arejiie outcome of terrestrial 

 fers. ^Tfr Lllualfgfe*aic means whereby the evolution was 

 initiated, Chamberlin 1 has discussed the peculiar habit of a 

 stream-borne lamprey, Petromyzon (Fig. 16, E, left-hand eel) , 

 which adheres to the bottom by its suctorial mouth and allows 

 its body to undulate in the pulsating current as a flag is 

 whipped in the breeze or a rope of grass in the stream. Unless 

 the stream be very shallow so as to cause distinct vertical dis- 

 placements of the water, this undulation is always lateral or 



1 Chamberlin, T. C., On the habitat of the early vertebrates. Jour. Geology, 

 vol. 8, 1900, pp. 400-412. 



