44 Physiology of the Kidney 



become extinct, we can expect to find among the surviving 

 organisms some that are much better fitted to endure severe 

 environmental changes than was the parent form. It is only 

 here, in the accidental development of increased indepen- 

 dence of environment, of increased physiological freedom, in 

 Bernard's sense of the word, that we can speak of evolution 

 as being upwards, rather than just sideways. 



The paleontological record reveals that evolution has not 

 been a continuous process, but an intermittent one. In 

 Lull's^^'" descriptive terms, it has been a tide of organic spe- 

 cialization moving forward in marked pulsations invariably 

 synchronous with the great upheavals of the earth's crust. It 

 was probably one of these pulsations, synchronous with the 

 Cambrian Revolution, that gave the vertebrates their start. 

 The more important steps in the phylogenetic history of these 

 forms, with special reference to those events that have a close 

 bearing upon the evolution of the kidney, are depicted graph- 

 ically in Figure 1. 



The problem of the origin of the first chordates remains 

 more or less where it was left by the great biologists of the 

 past century — in a sadly unsatisfactory state. A few years 

 ago there was consensus of opinion on at least one point: 

 that the chordates shared with the echinoderms, the acorn 

 worms, Branchiostoma (Amphioxus) and the tunicates, a 

 common marine ancestor, a frail-bodied, ghostly form, simi- 

 lar perhaps to the Dipleurula lava of the echinoderms.^"^ The 

 most important features of this hypothetical ancestor were 

 that it possessed a bilateral symmetry comparable to that of 

 Branchiostoma, and like Branchiostoma it kept one end fore- 

 most as it swam slowly and feebly through the archaic seas. 

 But the right of this ghostly form, the like of which no one 

 has ever seen, to spawn the vertebrate phylum has been re- 



