The Evolution of the Kidney 45 



cently questioned on the ground that the chordates, as they 

 first appear in the fossil record, were depressed, bottom liv- 

 ing, heavily armored and sluggish animals as far removed in 

 appearance from Branchiostoma as one can imagine. This 

 fact is in part responsible for the suggestion of Torsten Gis- 

 len^° which has been seconded by Gregory^^'^^ that the first 

 chordates may have been evolved from a free-swimming pal- 

 eozoic crinoid, or sea-lily. To this perennial debate we will 

 add one more confusing argument of our own in a later para- 

 graph. 



As we cannot say from what forms the first chordates 

 were evolved, neither can we with any certainty name the 

 time of their evolution. Some would assign this evolution to 

 the Ordovician Period, and some to the Cambrian. The open- 

 ing of the Cambrian was marked by one of the most violent 

 periods of mountain building the earth has ever known. 

 These mountains have long since been washed away, but the 

 sediment to which they were reduced is to be seen in the 

 several vertical miles of red and yellow banded rocks through 

 which the Colorado River has cut the Grand Canyon, and 

 from which scenic chasm the geologic revolution takes its 

 name. The biologist has repeatedly asserted that the truly 

 unique features of the vertebrates consist, broadly speaking, 

 of bilateral symmetry; of a stiffened and yet flexible internal 

 backbone with an articulated skeleton for the support of 

 muscles so arranged as to produce powerful lateral motions 

 of the body, the backbone, skeleton and muscles being made 

 up of regularly repeated segments; of paired, fin-like expan- 

 sions of the skin to resist the thrust of these muscles and to 

 maintain an even keel as the animal shoves itself forward in 

 the water; and, of course, the major sense organs are located 

 in the anterior end of the body. These features are just such 



