vj OF ASYMMETRY AND ANISOTROPY 245 



Unicellular organisms in general, including the protozoa, the 

 unicellular cryptogams, the various bacteria, and the free, 

 isolated cells, spores, ova, etc. of higher organisms, are referable 

 for the most part to a very small number of typical forms ; but 

 besides a certain number of others which may be so referable, 

 though obscurely, there are obviously many others in which 

 either no symmetry is to be recognized, or in which the form is 

 clearly not one of equihbrium. Among these latter we have 

 Amoeba itself, and all manner of amoeboid organisms, and also 

 many curiously shaped cells, such as the Trypanosomes and various 

 other aberrant Infusoria. We shall return to the consideration of 

 these ; but in the meanwhile it will suffice to say that, as their 

 surfaces are not equilibrium-surfaces, so neither are the living 

 cells themselves in any stable equilibrium. On the contrary, they 

 are in continual flux and movement, each portion of the surface 

 constantly changing its form, and passing from one phase to 

 another of an equilibrium which is never stable for more than 

 a moment. The former class, which rest in stable equilibrium, 

 must fall (as we have seen) into two classes, — those whose equi- 

 librium arises from liquid surface-tension alone, and those in 

 whose conformation some other pressure or restraint has been 

 superimposed upon ordinary surface-tension. 



To the fact that these httle 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. These forms vary according 

 to varying physical conditions. Sometimes they do so in so regular 

 and orderly a way that we instinctively explain them merely as 

 "phases of a life-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 suppose to be the typical or "characteristic" form or attitude. 

 In the case of the smallest organisms, the 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 



