They increase with such amazing rapidity that, unless limited by want of nutriment and favorable environment, a single species would in a few years occupy the earth's surface to the exclusion of all other life.
Many species are of wonderful vitality and tenacity of life, and resist the extremes of temperature, of boiling and freezing. Others may be dried to an entire suspension of vitality for months and years; wafted here and there by the winds until, under favoring circumstances, they renew their wonted activity. Scattered on the snow of the hill-side, and carried down with the spring freshet, miles away, they may be swallowed with the water by some unfortunate individual, and perhaps prove their presence and their source by inducing in him an infectious disease of their specific kind.
The microbes may be captured, and cultivated on beds of gelatin, albumen, sugar, and in broth of meats; and under skillful management be made to furnish flourishing colonies and specimens of the highest degree of development. Or they may be starved and chilled to such helpless weakness and attenuation as to seem to lose their specific characteristics. Those we are considering are among the most minute bodies within the possible scope of microscopy. This, and their perfect transparency, have heretofore seemed an insurmountable hindrance to our further knowledge of them. But the discovery of their strong affinity for certain of the intense coloring matters has been fortunate and timely, furnishing a key to brilliant developments, since it is found that certain species show a predilection for special colors; and a particular part of the microbe, as its membrane, or its contents, may unite with the color, while other parts may totally reject it—thus giving, not only outlines, but illuminated internal structure, otherwise invisible and unknown.
The extreme minuteness, then, of these bodies has heretofore been the bar and hindrance to our better knowledge of them. But already we have been able to peer downward and inward, from gross visible matter, through organs, tissues, cells, nuclei, nucleoli, and granules, until, in the so-called structureless protoplasm, our present hunting-ground and limit, we seem to have reached the confines of the inorganic molecule and atom, which are subject to chemical instead of physiological law. The modern discoveries in this microcosmic realm, and the demonstration of the causative relation of micro-organisms to disease, upon which the "germ theory of disease" depends, stands so conspicuously as a scientific success, and is a step so important toward the alleviation of suffering, the prolongation of life, and enhancement of human happiness as to be the subject of universal congratulation.
In order to comprehend the importance of this subject, it must
