52 Physiology of the Kidney 



— glucose, chloride, phosphate, etc., — for if these did not 

 pass through the filtering bed the great osmotic pressure 

 which they exert would effectively prevent the heart from 

 pumping any water through this bed. Hence, with the ad- 

 vent of the glomerulus it was necessary so to modify the 

 tubules that they could reabsorb these valuable constituents 

 from the filtrate. Moreover, there was such an excess of 

 water over salt to be excreted that the urine had to be almost 

 pure water, i.e., it had to have a substantially lower osmotic 

 pressure than the blood. Thus, as a concomitant of the evo- 

 lution of the glomerulus, there came into existence a tubule 

 capable of reabsorbing large quantities of glucose and similar 

 valuable substances, and capable of elaborating, by the reab- 

 sorption of salt, a urine that was hypotonic to the blood. 



To whatever extent this new fresh water kidney was ade- 

 quate to its time, times changed. The restless earth began to 

 heave again. At the close of the Silurian another diastrophic 

 movement disturbed its crust; no great mountains were 

 raised in North America, but a ridge higher than the Alps 

 was wrinkled up in Northern Europe, of which the low Cal- 

 endonian mountains of Scotland are all that now remain. 

 Other continental areas were extensively submerged beneath 

 the sea, and what land escaped was plagued by extremes of 

 climate swinging between excess of rain and drought. The 

 fishes of the early and middle Devonian found themselves 

 forced to choose between the invading salt water marshes and 

 the isolated fresh-water pools which periodically contracted 

 into stagnant swamps or hard mud flats. Some of the more 

 powerful elasmobranchs, perhaps now better fitted to com- 

 pete with the cephalopods and other marine invertebrates, 

 sought sanctuary by turning towards the sea; the fate of 

 these, the first fishes to live in salt water, will be noted in a 



