292 A NOTE OF ADSORPTION [ch. vi 



a fluid, we are resting our belief on a general consensus of evidence, 

 rather than on compliance with any one crucial definition. The 

 simple fact is that the agreement of cell-forms with the forms 

 which physical experiment and mathematical theory assign to 

 liquids under the influence of surface tension, is so frequently and 

 often so typically manifested, that we are led, or driven, to accept 

 the surface tension hypothesis as generally applicable and as 

 eqmvaient to a universal law. The occasional difficulties or 

 apparent exceptions are such as call for further enquiry, but fall 

 short of throwing doubt upon our hypothesis. Macallum's 

 researches introduce a new element of certainty, a "nail in a sure 

 place," wnen tliey demonstrate that, in certain movements or 

 changes of form which we should naturally attribute to weakened 

 surface tension, a chemical concentration which would naturally 

 accompany such weakening actually takes place. They further 

 teach us that in the cell a chemical heterogeneity may exist of 

 a very marked kind, certain substances being accumulated here 

 and absent there, within the narrow bounds of the system. 



Such localised accumulations can as yet only be demonstrated 

 in the case of a very few substances, and of a single one in par- 

 ticular ; and these are substances whose presence does not produce, 

 but whose concentration tends to follow, a weakening of surface 

 tension. The physical cause of the localised inequalities of surface 

 tension remains unknown. We may assume, if we please, that it 

 is due to the prior accumulation, or local production, of chemical 

 bodies which would have this direct effect; though we are by 

 no means limited to this hypothesis. 



But in spite of some remaining difficulties and uncertainties, 

 we have arrived at the conclusion, as regards unicellular organisms, 

 that not only their general configuration but also their departures 

 from symmetry may be correlated with the molecular forces 

 manifested in their fluid or semi-fluid surfaces. 



