Thus it will be seen that in this last dairy product bacteria play even a more important part than in any of the others.  The food value of cheese is dependent upon the casein which is present.  The market price, however, is controlled entirely by the flavour, and this flavour is a product of bacterial growth.  Upon the action of bacteria, then, the cheese maker is absolutely dependent; and when our bacteriologists are able in the future to investigate this matter further, it seems to be at least possible that they may obtain some means of enabling the cheese maker to control the ripening accurately.  Not only so, but recognising the great variety in the flavours of cheese, and recognising that different kinds of bacteria undoubtedly produce different kinds of decomposition products, it seems to be at least possible that a time will come when the cheese maker will be able to produce at—­ will any particularly desired flavour in his cheese by the addition to it of particular species of bacteria, or particular mixtures of species of bacteria which have been discovered to produce the desired effects.

CHAPTER IV.

Bacteria in natural processes.—­Agriculture.

Thus far, in considering the relations of bacteria to mankind, we have taken into account only the arts and manufactures, and have found bacteria playing no unimportant part in many of the industries of our modern civilized life.  So important are they that there is no one who is not directly affected by them.  There is hardly a moment in our life when we are not using some of the direct or indirect products of bacterial action.  We turn now, however, to the consideration of a matter of even more fundamental importance; for when we come to study bacteria in Nature, we find that there are certain natural processes connected with the life of animals and plants that are fundamentally based upon their powers.  Living Nature appears limitless, for life processes have been going on in the world through countless centuries with seemingly unimpaired vigour.  At the very bottom we find this never-ending exhibition of vital power dependent upon certain activities of micro-organisms.  So thoroughly is this true that, as we shall find after a short consideration, the continuance of life upon the surface of the world would be impossible if bacterial action were checked for any considerable length of time.  The life of the globe is, in short, dependent upon these micro-organisms.

Bacteria as scavengers.