516 The American Naturalist. [June, 
duced beneath the skin would by some be called an abnormal 
function performed by the wandering cells. It would seem, how- 
ever, that a tissue so constructed as to be able to adapt itself to 
conditions caused by some unforeseen accident would employ an 
entirely normal function in adapting itself to new circumstances. 
An irritating particle may be placed between the shell and mantle 
of an oyster, and very soon the cells of the latter secrete a hard 
covering of pearl over the foreign object. The secretion would 
not have taken place had it not been for this accident, and yet we 
regard the process as an entirely normal one. In the case of the 
collecting of dust particles in the alveoli of the lungs, some may 
call it a natural function, while others may have a different 
opinion. The leucocytes in animal bodies, though they may 
differ from one another in some respects, are essentially alike in 
function as far as is known, and anything that they are called 
upon to do, and are able to perform, may be regarded as normal. 
One more fact concerning the action caused by the introduc- 
tion of foreign, inorganic bodies is that, if the substance be too 
large, the leucocytes often unite with one another and cover the 
object. They make a fixed covering of what is called fibrous 
tissue, and the process is known as encystment. Sometimes, 
however, great numbers of leucocytes may die in the attempt to 
dispose of foreign particles, and their disintegrated bodies form 
a substance called pus. The sore resulting from this is an — 
abscess. 
We shall now see that these leucocytes have a far more inter- 
esting and important function than those already mentioned, and 
its discovery was made by Metschnikoff. It is known to every 
one that a number of diseases to which man and other mam- 
mals are subject are caused by bacteria,—microscopic plants,— 
which enter the body in various ways, and by their multiplication 
occasion changes in the tissues that may be exceedingly danger- 
ous to the attacked individual. It'is claimed by many, that the 
most important function of the phagocytes is to take up these 
micro-organisms and destroy them. 
Perhaps the most interesting observation given us on this sub- 
ject by Metschnikoff is that made upon Daphnia, a small fresh- 
