92 INTRODUCTION TO EVOLUTION 



water, so animals living in water readily lose their ammonia to it. Hence, 

 a large proportion of the nitrogenous wastes of aquatic animals is excreted 

 as ammonia. Among vertebrates this is true of most fishes, which excrete 

 ammonia mainly through their gills. 



The land dwellers cannot rid themselves of ammonia so easily. Accord- 

 ingly we find that the amphibians have a different method of dealing with 

 the problem. They convert the ammonia into urea (by combining it with 

 carbon dioxide). Urea is readily soluble in water and differs from ammonia 

 in being relatively nontoxic. Thus it can be stored in the body and even- 

 tually excreted by the kidney in the urine. 



It will be recalled that typical amphibians, such as frogs, have a life his- 

 tory which includes a tadpole stage. The tadpole is a larva living in the 

 water, using gills for respiration, and in general leading a fishlike exist- 

 ence. Interestingly enough, frog tadpoles also resemble fishes in excreting 

 most of their nitrogenous wastes in the form of ammonia. Then, upon 

 metamorphosis into the adult form, the change to the urea-forming mode 

 of excretion is made. Here evidently is an example of recapitulation (see 

 Chap. 4), in this case biochemical recapitulation. 



A slightly more complex situation is presented by the vermillion-spotted 

 newt (Triturus). Nash and Fankhauser (1959) found that young larvae, 

 living in the water, excrete most of their nitrogen as ammonia but that the 

 proportion so excreted decreases as time for metamorphosis approaches. 

 By the time of metamorphosis about 80 percent of the nitrogen is excreted 

 as urea, and the proportion increases to 87 percent during terrestrial life 

 (the "red eft" stage). Eventually the newts return to the water to live as 

 adults. In these aquatic adults partial return to the larval pattern of ex- 

 cretion occurs: 26 percent of the nitrogen is excreted as ammonia, 74 per- 

 cent as urea. 



Passing on to more completely terrestrial vertebrates, we find reptiles, 

 birds, and mammals faced with the problem of conserving the body's wa- 

 ter supply. How can the body be rid of nitrogenous waste products with- 

 out undue loss of water in the process? There are two general solutions 

 to the problem. Most mammals have retained the process of converting 

 ammonia into urea, which is conveyed by the blood to the kidney. The 

 tubules of the mammalian kidney have developed a special portion 

 (Henle's loop) which serves the purpose of removing water from the 

 urine, leaving the latter highly concentrated. Mammalian urine, then, con- 

 tains a maximum of waste carried by a minimum of water. 



Birds and most reptiles solve the problem somewhat differently. They 

 convert ammonia into uric acid, an almost insoluble compound. The urine 



