EVOLUTION AS SEEN IN SEROLOGICAL TESTS 113 



apes. Or possibly the changes in serum proteins in these latter two lines 

 have been of similar nature ahhough independent in occurrence, affording 

 an example of parallel evolution (see p. 29). 



If the evolutionary explanation for the similarity in serum structure is 

 the correct one, serological tests form a means of measuring the degree 

 of relationship of animals to one another. The principle involved is that 

 the degree of chemical similarity is proportional to the degree of relation- 

 ship. Closely related forms will have serum proteins much alike; more dis- 

 tantly related forms will have serum proteins less alike. On the whole, tests 

 actually performed seem to support this principle, despite the possibility 

 that occasional similarity between sera may be due to convergent or paral- 

 lel evolution, as some similarities in morphology are (Chap. 3). Moreover, 

 it is to be noted in passing that parallel evolution may indicate relation- 

 ship of the forms concerned. Evidence shows that the nature of serum pro- 

 teins is controlled by the units of heredity, the genes. We have noted that 

 genes occasionally undergo chemical alteration (mutation). The variety of 

 mutations possible to any one kind of gene is limited. Thus if two forms 

 inherit a certain gene from a common ancestor that gene may later un- 

 dergo the same mutation in both forms, the resultant changes being similar 

 in nature though independent in occurrence. Thus parallel evolution may 

 indicate possession by the forms concerned of similar or identical genes, 

 inherited from a common ancestor. The importance of such independent 

 recurrence of mutations in different though related forms receives addi- 

 tional emphasis in our discussion of the cell proteins determining the blood 

 groups (pp. 121-125). 



Measuring the Precipitin Test 



If serological tests are to be utilized for measuring animal relation- 

 ships, some method of measuring the precipitate formation must be 

 adopted. Various methods have been employed. The volume of precipitate 

 settling to the bottom of the tube may be measured in a special calibrated 

 tube of small diameter. Or the Kjeldahl test may be employed to deter- 

 mine the amount of nitrogen present in the precipitate — the amount of 

 nitrogen being proportional to the amount of precipitate. The most widely 

 used method consists of making serial dilutions of the serum (antigen) to 

 be tested (Fig. 6.2). Our diagram is based on data obtained in a study of 

 the relationships of a number of mammals to the sheep. Inoculation of a 

 rabbit with sheep serum causes production of antibodies against sheep 

 serum. A small quantity of the rabbit serum containing antibodies against 



