THE WONDER OF LIFE 509 



Individuality of the Blood. What is called the serum 

 test for blood is a good illustration of the subtle individu- 

 ality of different creatures. If the serum of human blood 

 is injected into a rabbit, it produces a change in the rabbit's 

 blood of a very specific kind. As de Nobele showed in 

 1902, the serum of that rabbit will give a precipitate with 

 human blood, but not with the blood of other Mammals. 

 Thus if a murderer asserts that the bloodstains on his 

 clothes are due to his having killed a rabbit, not a man, 

 his statement can be tested ; and the method has passed 

 into the ordinary practice of medical jurisprudence. The 

 serum for testing with can be kept for months in a dry state 

 (after evaporation in a vacuum) without losing its relia- 

 bility, and bloodstains that are several months old may 

 be accurately identified. Of course the method has been 

 tested and re-tested hundreds of times, and little improve- 

 ments in detail have been introduced. It should be 

 noticed that the principle of the method was discovered 

 in relation to milk, and that it was applied in the identifica- 

 tion of different kinds of milk and different kinds of flesh 

 before it was applied to blood. 



ADAPTATION 



Wherever we look throughout the wide world of animate 

 nature, we find illustrations of particular fitness to parti- 

 cular conditions. The size, the shape, the colour of an 

 organism, the structure of parts in relation to their use 

 and in their relation to other parts all are adaptive. In 

 the same way the characteristic behaviour of the creature 

 in its everyday life, and the internal activities within the 

 body all are adaptive. And what is true of everyday 



