60 



THE ORIGINATION OF PARASITISM. 



liquid, such as water, when allowed to rest upon a rough surface which 

 it will not wet. Under equable external conditions such a drop would 

 be approximately g-lobular, with all of its sectors practically equivalent 

 (fig'. 2, .-/). When placed upon a roughened surface the shape of the drop 

 would be altered as shown in fig. 2. />', the sectors in contact with the hard 

 external body would be markedly modified, the position of the centroid 

 would not be the same as that of the drop originally, and the non-engaged 

 sectors would be altered to a degree corresponding to the directness of 

 their connection with those in contact with the disturbing objects. 



'ig. 2.--A, diagram showing general equivalence of sectors of a drop of liquid 

 in a homogeneous medium. B, condition of drop when broughtinto contact 

 with a rough surface, showing greatest distortion in sectors in direct contact, 

 and least in others by correlation stresses. 



With the further application of this illustration the study of the mode of 

 adjustment of an organism to an environmental factor would entail a de- 

 termination of the changes in the engaging sectors or characters directly 

 modified, the measurement of the effects of the external factors, an estima- 

 tion of the limiting conditions, and the detection of connected variations in 

 the other sectors, functions, or qualities of the living drop. Among the 

 many other suggestive parallels that might be drawn is to be mentioned 

 the mechanical fact that under the altered conditions of surface-tension 

 ensuing from contact with one set of hard objects, the drop is much more 

 liable to be changed further or to be broken up by other agencies acting 

 upon the free or unengaged surfaces. 



