522 The American Naturalist. [June, 
are now being suggested. Some believe that certain diseases 
may be caused by a combination of many kinds of micro- 
érganisms. The opinion seems to be growing that ordinarily harm- 
less, or septic, forms may, under favorable conditions, change into 
those which are harmful, or pathogenic. 
It is probable that most people, or at least those living in great 
cities, take into their bodies daily, micro-organisms which, under 
very favorable conditions, would cause disease. One person may . 
take in safely, perhaps, many times the number required to cause 
disease in another. Sutton has expressed this fact as follows: 
“ The more these questions are studied, the more we perceive that 
the outbreak of infectious diseases depends not so much on the 
presence of microorganisms—for, like torula (the yeast plant), 
they seem to exist everywhere—as upon the existence of suitable 
_conditions, and as yeast cannot grow and multiply without sugar, 
neither can the poison of erysipelas, typhus fever, and the like 
propagate without the presence of some substance produced in 
living bodies, of the nature of which we are ignorant.” For 
example, “ relapsing fever is unknown except in times of famine, 
when the body-chemistry is deranged by want of food, privation, 
and hardships of every kind.” 
But to return to the phagocytes, we must notice a few objec- 
tions that have been urged against the experiments of Metschni- 
koff. A few investigators who do not believe in the phagocyte 
theory, claim that there are other elements in the body which 
exert an active influence against microorganisms ; but, of course, 
this, in itself, is no argument against the supposed function of the 
wandering cells. Ribbert, and one or two others, while agreeing 
with Metschnikoff on some points, believe from their experiments 
that such fungi as microdrganisms, or spores of fungi, are pre- 
vented from growing in a tissue, not so much on account of 
an active attack of leucocytes, as the fact that the latter probably 
deprive them of oxygen necessary to their growth, and perhaps 
also keep away other nourishing materials. It seems to be impos- 
sible for any one to contradict, in any case, the fact that the pha- 
gocytes take up bacteria, unless in the instance referred to of splenic 
fever, when in the blood, this does not occur, as Metschnikoff 
