EVOLUTION AS SEEN IN SEROLOGICAL TESTS 117 



serological tests have here helped to fill a gap in our knowledge of relation- 

 ships. 



Rabbits and Rodents 



Similar uncertainty exists concerning the relationships of hares and rab- 

 bits to other mammals. These animals are like rodents (rats, mice, squir- 

 rels, beavers, woodchucks, etc. ) in possession of a gnawing arrangement 

 of the front or incisor teeth. Two upper incisors are greatly enlarged so 

 that they resemble small chisels. A similar pair of chisel-Hke lower incisors 

 meets the upper pair to make possible efficient gnawing. Because hares 

 and rabbits, on the one hand, and rodents, on the other, possess this ar- 

 rangement the two groups were for many years both included in Order 

 Rodentia. The practice widely adopted today, however, is to emphasize 

 the many morphological features by which rabbits differ from rodents, and 

 accordingly to place hares and rabbits in the separate Order Lagomorpha. 

 Even today, however, the separation of lagomorphs and rodents into sepa- 

 rate orders is not always followed, despite the fact that no forms inter- 

 mediate between lagomorphs and rodents are known from the fossil rec- 

 ord, and that representatives of both orders are known from as ancient 

 times as the Paleocene (p. 137), these ancient representatives being as dis- 

 tinctly rodents, on the one hand, and lagomorphs, on the other, as are 

 their modern descendants. 



Among biologists who agree to the separation of Order Lagomorpha 

 from Order Rodentia there is disagreement as to whether or not lago- 

 morphs are more closely related to rodents than they are to some other or- 

 ders of mammals. For example, it has long been recognized that lago- 

 morphs resemble artiodactyls in some morphological features. 



It occurred to the author that this was a question upon which serological 

 tests might shed light. Accordingly with his graduate students he carried 

 on an investigation employing both the ring tests and the turbidity (pho- 

 tronreflectometer) tests described above. Since the rabbit was to be one 

 of the subjects of investigation it could not also be employed as antibody 

 producer. Male domestic fowl were found to serve this purpose admirably. 

 Being birds, they may be considered equally removed from all mammals 

 and hence in a position to afford the proper "serological perspective." 



An example of results obtained with the turbidity test is presented in 

 Fig. 6.3. The numbers along the base of the graph represent the tubes in 

 the series of dilutions. Tube 1 contained whole serum as antigen. Tube 2 



