CHAPTER IX 



EVIDENCE FROM BLOOD TESTS' 

 W. B. Scott 



Here may be conveniently considered the very interesting and 

 significant blood tests which have been made in the last fifteen years 

 by various physiologists and especially by Dr. George H. F. Nuttall, 

 of the University of Cambridge. Though there are several methods 

 of making these tests, the "precipitation method" employed by 

 Dr. Nuttall will be quite sufficient for the ends sought in these lec- 

 tures. 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 showoi by the fact that, in performing the 

 operation for the transfusion of blood, it was not practicable to 

 substitute animal for human 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 \y 

 will do in a few minutes, 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 several days and is then 

 killed and bled, the blood is left until it clots and the serum drained 

 off and preserved. The serum obtained thus from a rabbit is called 

 "anti-human" serum and is an exceedingly delicate test for human 

 blood, not only when the latter is fresh, but also when it is in 

 the form of old and dried blood-stains, or even when the blood is 



^ From W. B. Scott, Tlie Theory of Evolution (copyright 191 7). Used by 

 special permission of the publishers, The Macmillan Company. 



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