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 
