and their ability to produce profound chemical changes is therefore unlimited.  This vast host of living beings thus constitutes a force or series of forces of tremendous significance.  Most of the vast multitude we must regard as our friends.  Upon them the farmer is dependent for the fertility of his soil and the possibility of continued life in his crops.  Upon them the dairyman is dependent for his flavours.  Upon them important fermentative industries are dependent, and their universal powers come into action upon a commercial scale in many a place where we have little thought of them in past years.  We must look upon them as agents ever at work, by means of which the surface of Nature is enabled to remain fresh and green.  Their power is fundamental, and their activities are necessary for the continuance of life.  A small number of the vast host, a score or two of species, unfortunately for us, find their most favourable living place in the human body, and thus become human parasites.  By their growth they develop poisons and produce disease.  This small class of parasites are then decidedly our enemies.  But, taken all together, we must regard the bacteria as friends and allies.  Without them we should not have our epidemics, but without them we should not exist.  Without them it might be that some individuals would live a little longer, if indeed we could live at all.  It is true that bacteria, by producing disease, once in a while cause the premature death of an individual; once in a while, indeed, they may sweep off a hundred or a thousand individuals; but it is equally true that without them plant and animal life would be impossible on the face of the earth.