550 



Rabbit serum 



which contains anti 



Precipitate 



Horse '. 

 serum » 



bodies against horse serum 



The Earth mid Its inhabitants Change unit x 



Fig. 503 Into A was put horse serum; 

 into B was put donkey serum; and 

 into C was put dog serum. Then 

 serum frojn a rabbit which had 

 for?}ied antibodies against horse serum 

 after inocidation was added to each 

 tube. Note the precipitate which 

 jorms in A. Compare with B and with 

 C. IVhat does this experiment seem 

 to show about the closeness of rela- 

 tionship of horse and donkey as com- 

 pared with horse and dog? Explain. 



Donkey 

 I serum* 



' Dog I 

 serum I 



is strikingly like that of the lower verte- 

 brates. It only gradually comes to look 

 like a human baby. Compare Figures 501 

 and 502. These similarities do not seem 

 strange if we assume that all vertebrates 

 are descendants of a common ancestor. 

 If they are related to each other, it is not 

 surprising that their development should 

 be similar. Exercise 2 will help in sum- 

 ming up this paragraph. 



Chemical similarities. You already 

 know that all living cells, from the 

 simplest to the most complex, are much 

 alike. All get energy by oxidation of 

 food; all consist of protoplasm contain- 

 ing, among other substances, water, car- 

 bohydrates, fats, and proteins; all pro- 

 duce enzymes of one kind or another. 

 These things are true of both plants and 

 animals. If we assume a common ances- 

 tor for both, these facts are not too diffi- 

 cult to understand and explain. 



More striking chemical similarities 

 exist. Those forms which are alike in 

 structure are also frequently alike in 

 chemical make-up. Only vertebrates 



have hemoglobin in their blood cells. 

 Only those plants which, because of 

 their structural similarity, are classified 

 in the same genus (lemons, limes, or- 

 anges, grapefruit) produce citric acid. 

 These chemical resemblances would be 

 difficult for us to explain did we not 

 believe that the organisms which show 

 these similarities are descended from 

 common ancestors. 



In delicate tests with serums and anti- 

 bodies it is found that chemical resem- 

 blances are greatest in those forms which 

 seem from their characteristics to be 

 most closely related. Figure 503. Exer- 

 cise 3 will test your understanding. 



How can apparently useless structures 

 be explained? Alan has an appendix which 

 seems to be of no use and may do harm. 

 Some people have scalp and ear muscles 

 which can be used, though to no pur- 

 pose. The bones at the lower end of the 

 spinal column in man are small and 

 fused; as you learned, they form a short 

 tail. The whale has no hind legs, but it 

 carries through life a number of small 



