V] OF STABLE AND UNSTABLE EQUILIBRIUM 405 



surface constantly changing its form, passing from one phase to 

 another of an equihbrium which is never stable for more than a 

 moment, and which death restores to the stable equilibrium of a 

 sphere. The former class, which rest in stable equihbrium, must 

 fall (as we have seen) into two classes — those whose equilibrium 

 arises from liquid surface-tension alone, and those in whose con- 

 formation some other pressure or restraint has been superimposed 

 upon ordinary surface-tension. 



To the fact that all these organisms belong to an order of 

 magnitude in which form is mainly, if not wholly, conditioned and 

 controlled by molecular forces is due the hmited range of forms 

 which they actually exhibit. They vary according to varying 

 physical conditions. Sometimes they do so in so regular and 

 orderly a way that w^e intuitively explain them as "phases of a 

 Hfe-history," and leave physical properties and physical causation 

 alone: but many of their variations of form we treat as exceptional, 

 abnormal, decadent or morbid, and are apt to pass these over in 

 neglect, while we give our attention to what we call a typical or 

 "characteristic" form or attitude. In the case of the smallest 

 organisms, bacteria, micrococci, and so forth, the range of form is 

 especially limited, owing to their minuteness, the powerful pressure 

 which their highly curved surfaces exert, and the comparatively 

 homogeneous nature of their substance. But within their narrow 

 range of possible diversity these minute organisms are protean in 

 their changes of form. A certain species will not only change its 

 shape from stage to stage of its little "cycle" of hfe; but it will 

 be remarkably different in outward form according to the circum- 

 stances under which we find it, or the histological treatment to 

 which we subject it. Hence the pathological student, commencing 

 the study of bacteriology, is early warned to pay little heed to 

 differences of form, for purposes of recognition or specific identi- 

 fication. Whatever grounds we may have for attributing to 

 these organisms a permanent or stable specific identity (after 

 the fashion of the higher plants and animals), we can seldom 

 safely do so on the ground of definite and always recognisable 

 form: we may often be inclined, in short, to ascribe to them a 

 physiological (sometimes a "pathogenic") rather than a morpho- 

 logical specificity. 



