74 THE THEORY OF EVOLUTION 



of the University of Cambridge. Though there are 

 several methods of making these tests, the "precipi- 

 tation method" employed by Dr. Nuttall will be 

 quite sufficient for the ends sought in these lectures. 

 The method and significance of the tests can best be 

 explained by taking as an example human blood, 

 which, of course, has been most extensively and 

 minutely studied, because of its legal importance as 

 well as its scientific interest. Ordinary chemical 

 analysis is unable to determine the differences in 

 blood-composition between various animals, but 

 that there were important differences had long been 

 understood. This was shown by the fact that, in 

 performing the operation for the transfusion of blood, 

 it was not practicable to substitute animal for hu- 

 man blood, since the former might cause serious 

 injury to the patient. 



The precipitation method of making blood tests 

 is as follows: Freshly drawn human blood is allowed 

 to coagulate or clot, which it will do in a few min- 

 utes, if left standing in a dish, and then the serum 

 is drained away from the clot. Blood-serum is the 

 watery, almost colourless part of the blood, which 

 remains after coagulation. Small quantities of this 

 serum are injected, at intervals of one or two days, 

 into the veins of a rabbit and cause the formation 

 in the rabbit's blood of an anti-body, analogous to 

 the anti-toxin which is produced in the blood of a 

 horse by the injection of diphtheria virus. After 

 the last injection the rabbit is allowed to live for 



