V] 



OF VARIOUS UNDULOIDS 



249 



FoUiculina. 



present, and has its actual conformation, by reason of certain 

 chemico-physical conditions : that it was inevitable, under the 

 given conditions, that certain constituent 

 substances actually present in the proto- 

 plasm should be aggregated by molecular 

 forces in its surface layer ; that under this 

 adsorptive process, the conditions con- 

 tinuing favourable, the particles should 

 accumulate and concentrate till they 

 formed (with the help of the surrounding 

 medium) a pellicle or membrane, thicker 

 or thinner as the case might be ; that this 

 surface pellicle or membrane was inevitably bound, by molecular 

 forces, to become a surface of the least possible area which the 

 circumstances permitted ; that in the 

 present case, the symmetry and " freedom " 

 of the system permitted, and ipso facto 

 caused, this surface to be a surface of 

 revolution ; and that of the few surfaces 

 of revolution which, as being also surfaces 

 ?ninimae areae, were available, the undu- 

 loid was manifestly the one permitted, 

 and ipso facto caused, by the dimensions 

 of the organisms and other circumstances 

 of the case. And just as the thickness or 

 thinness of the pellicle was obviously a 

 subordinate matter, a mere matter of 

 degree, so we also see that the actual 

 outline of this or that particular unduloid 

 is also a very subordinate matter, such as 

 physico-chemical variants of a minute kind 

 would suffice to bring about ; for between 

 the various unduloids which the various 

 species of Vorticella represent, there is no 

 more real difference than that difference 

 of ratio or degree which exists between 

 two circles of different diameter, or two 

 lines of unequal length. 



Fig. 80. Trachelophyllum. 

 (After Wreszniowski.) 



