116 INTRODUCTION TO EVOLUTION 



are clear and firmly established gives confidence in the validity of serologi- 

 cal results in cases where morphology does not by itself afford clear and 

 unequivocal evidence as to relationships. 



The whales afford a case in point. To which of the terrestrial mammals 

 are they most closely related? Simpson (1945) states concerning them, 

 "Because of their perfected adaptation to a completely aquatic life, with 

 all its attendant conditions of respiration, circulation, dentition, locomotion, 

 etc., the cetaceans are on the whole the most peculiar and aberrant of 

 mammals. Their place in the sequence of cohorts and orders is open to 

 question and is indeed quite impossible to determine in any purely objec- 

 tive way." Perhaps serological tests may afford the objective determination 

 sought. Nuttall inoculated a rabbit with whale serum and used the re- 

 sultant antiserum in tests with sera from a considerable array of mammals. 

 Strongest reactions were obtained with the sera of other whales. Some re- 

 actions of medium strength occurred with sera from representatives of Or- 

 der Artiodactyla — even-toed mammals. Sera of other mammals gave less 

 reaction or no reaction at all. 



Tests by Boyden and Gemeroy (1950) strengthened the evidence of 

 serological relationship between whales and artiodactyls. These investiga- 

 tors employed a modification of the serial-dilution method described 

 above (Fig. 6.2). In each tube the antigen dilution was mixed with the 

 antiserum instead of being overlayered above it. The amount of turbidity 

 or cloudiness which developed in each tube in 20 minutes as a result of 

 antigen-antibody reaction was measured with a photoelectric instrument 

 called the Libby Photronreflectometer. Such turbidity formation precedes 

 the settling of precipitate to the bottom of the tube as shown in Fig. 6.1. 

 With the instrument mentioned it is possible to measure the amount of re- 

 action in all the tubes of the series of dilutions. The measurement used in 

 statement of relationships is, accordingly, based on the sum of the reac- 

 tions in all the tubes, instead of being based merely upon the magnitude of 

 the greatest dilution in which a reaction (ring) can be seen, as in the inter- 

 facial test (Fig. 6.2). Using antisera prepared against whale serum, and 

 checking the results with reciprocal tests employing antisera prepared 

 against beef serum, Boyden and Gemeroy found clear indication that 

 whales are more similar serologically to artiodactyls than they are to any 

 other order of mammals. This might mean that whales sprang from primi- 

 tive artiodactyl stock or that both arose from the same ancestral Condy- 

 larthra (pp. 195-196). We shall not know until the fossil record of whale 

 evolution becomes more complete than it is at present. At any rate, 



