Variation in Bacteria.

The bearing of slight physiological differences upon the classification of bacteria and upon the phenomena of infection has made the occurrence of variation among bacteria fully as conspicuous as in the higher forms of life. During the past ten years many observations have been recorded upon the extent and nature of bacterial variability. In these studies some confusion has arisen through the difficulty of distinguishing true variations from the development of latent characteristics, and from environmental modification. By the term 'latent characteristics' is meant those qualities or properties that are dormant in the organism or cell and are manifested only in response to definite external influences. Thus, certain bacteria form spores in the presence, but not in the absence, of oxygen; some bacteria are known that develop conspicuous capsules when growing in the animal body, but lack these envelopes in part or altogether in artificial media; according to Wright, animal fluids seem to be essential for the production of the characteristic clubs of Actinomyces colonies. The sudden appearance in this way of a definite morphological character cannot be looked upon as an instance of'variation. Such a manifestation is merely an immediate response to changed conditions of life and is of exactly the same kind as the marked difference in the aquatic and terrestrial forms of Polygonum amphibium referred to by DeVries, or as shown in the transformation of the shrimp Artemia salina into what some writers consider another species, Artemia milhausenii, when the former is transferred to water of a greater degree of saltiness. In no sense is the awakening of such a dormant character to be confounded with true variation. In other words the power to produce certain structures or certain physiological effects exists ready formed in the specific 160