EVOLUTION-PAST AND PRESENT 



cause they all give strong reactions with 

 sera containing antibodies for the domestic 

 fowl. Paleontologists have long contended 

 that birds evolved from the line that gave 

 rise to turtles and crocodiles ratlier than the 

 snake-lizard line and blood tests have con- 

 firmed their findings. 



Another interesting evidence for evolu- 

 tion comes from a study of the salt content 



641 



Such studies show that the fluids (blood) of 

 modern animals are about as saline as those 

 of the ancient seas (0.55), which would be 

 expected if animals migrated onto land at 

 the time when the salt content was con- 

 siderably less than it is today (1.85). The 

 blood of a fish, such as the shark, which has 

 never left the ocean possesses about the 

 same salt content as modern seas. This is 



Fig. 25-16. Occasionally an Embryonic structure persists in the adult, such as a well-cievelopecJ tail or 

 "sTZstoT'T '" ,''7--/»^-^^»-ct"- are founcl in relatives of man and n,aXe existed Z 

 h.s ancestors. The only feasible explanation is that of common origins. 



of the ancient and modern seas as com- 

 pared with that of body fluids of ancient 

 and modern animals (Fig. 25-17). It is 

 known that the salt content of the seas has 

 steadily increased through the ages. Freez- 

 ing-point depression studies are a con- 

 venient method of determining indirectly 

 the number of particles of solute in a fluid, 

 for the greater the number of particles, the 

 greater the freezing point is depressed. 



likewise true of invertebrates such as crabs. 

 Certain fish such as the salmon, which sur- 

 vives in both fresh and salt water, show 

 freezing-point depressions of 0.74, which is 

 intermediate between salt and fresh-water 

 animals. The whale shows a figure of 0.58, 

 again indicating its mammalian affiliations, 

 even though today it lives in the ocean. This 

 striking physiological evidence corrobo- 

 rates the course followed by animals from 



