492 THE ORIGIN OF VERTEBRATES 



possible that further investigation into the nature and connection of 

 these " chromaffine " cells may afford a clue to the origin of the 

 sympathetic nervous system. At present it is premature to discuss 

 the question further. 



Finally, another test as to the kinship of two animals of different 

 species must be considered more fully than I have been able to do 

 up to the present time. This test is of a totally different nature to 

 any put forth in previous pages. It is known as the " biological 

 test " of relationship, and is the outcome of pathological rather than 

 of physiological or anatomical research. It is possible that this test 

 may prove the most valuable of all. At present we do not know 

 sufficiently its limitations and its sources of error, especially in the 

 case of cold-blooded animals, to be able to look upon it as decisive in 

 a problem of the kind considered in this book. 



The nature of this test is as follows : It has been found that the 

 serum of the blood of another animal, when injected in sufficient 

 quantity into a rabbit, will cause such a change in the serum of that 

 rabbit's blood that when it is added to the serum of the other animal 

 a copious precipitate is formed, although the serum of normal rabbit's 

 blood when mixed with that of another animal will cause no precipi- 

 tate whatever. This extraordinary production of a precipitate in the 

 one case and not in the other indicates the production of some new 

 substance in the rabbit's serum in consequence of the introduction of 

 the foreign serum into the rabbit, which brings about a precipitate 

 when the rabbit's serum containing it is mixed with the serum 

 originally injected. The barbarous name "antibody " has been used 

 to express this supposed substance in accordance with the meaning 

 of such a word as " antitoxin," which has been a long time in use in 

 connection with preventive remedies against pathogenic bacteria. 

 Now, it is found that the rabbit's serum containing a particular 

 " antibody " will cause a precipitate only when added to the serum 

 of the blood of the animal from which the " antibody " was produced 

 or to the serum of the blood of a nearly related animal. 



Further, if that animal is closely related a precipitate will be 

 formed nearly as copious as with the original serum, if more distantly 

 related a cloudiness will occur rather than a precipitate, and if the 

 relationship is still more distant the mixture of the two sera will 

 remain absolutely clear. Thus this test demonstrates the close 

 relationship of man to the anthropoid apes and his more distant 



