EVOLUTION AS SEEN IN SEROLOGICAL TESTS 115 



the end point of the reaction. In Fig. 6.2 the end point of the homologous 

 reaction (that with serum of the kind originally inoculated into the rabbit 

 — sheep serum in this instance ) is shown as being 1 : 25,600. Although this is 

 a hypothetical example, the results shown are in keeping with those of tests 

 actually performed. 



The figure shows a similar ring test of the reaction of antibodies against 

 sheep serum when tested with goat serum. A series of dilutions of goat 

 serum is prepared and overlayered above the antiserum in a series of 

 tubes, as was done with the sheep serum in the preceding test. In this case 

 the highest dilution in which a ring forms is indicated as being 1:12,800. 

 The ratio of the two end points gives a quantitative statement of the 

 amount of similarity in serum proteins and hence of the degree of relation- 

 ship of the two species. By custom the homologous end point ( 1 : 25,600 in 

 this case) is regarded as 100 percent. The end point obtained with the goat 

 serum (1:12,800) can then be expressed as 50 percent, a value slightly 

 lower than that usually obtained in actual tests of sheep-goat relationships. 



The test is repeated, using the same antiserum, with a series of dilutions 

 of beef serum. The end point with the latter is shown as being 1 :6400. This 

 corresponds to 25 percent of the homologous end point. Thus, insofar as 

 one can draw conclusions from a single series of tests, the goat is twice as 

 closely related to the sheep as is the beef animal. 



Tests utihzing dilutions of dog serum are shown as yielding an end point 

 of 1 : 1600 — 6.25 percent of the homologous end point. This indicates that 

 the dog is much less closely related to the sheep than the beef animal is. 



Serology Supplements Morphology 



The order of relationship indicated in the test just described (Fig. 6.2) is 

 in accordance with expectation based on morphological studies of the ani- 

 mals mentioned. This fact is reflected in the classification of the latter. The 

 sheep, goat, and beef are included in one order, Order Artiodactyla — even- 

 toed hoofed mammals. The dog, on the other hand, is a member of Order 

 Carnivora, along with most other flesh-eating mammals. 



Indeed, from the pioneer investigations of Nuttall ( 1904) to the present 

 most serological results have served to confirm the generally accepted 

 classification of animals based on morphology. This is what would be ex- 

 pected if both morphological and serological similarities are attributable 

 to inheritance from common ancestry. The fact that serological results con- 

 firm relationships as determined by morphology when these relationships 



