358 THE FORMS OF CELLS [ch. 



the component atoms is precisely that for which the energy is a 

 minimum; and we recognise that this minimal distril^ution is of 

 itself tantamount to symmetry and to stability. 



Moreover, the principle of least action is but a setting of a still 

 more universal law — that the world and all the parts thereof tend 

 ever to pass from less to more probable configurations; in which 

 the physicist recognises the principle of Clausius, or second law of 

 thermodynamics, and with which the biologist must somehow 

 reconcile the whole "theory of evolution." 



As we proceed in our enquiry, and especially when we approach 

 the subject of tissues, or agglomerations of cells, we shall have from 

 time to time to call in the help of elementary mathematics. But 

 already, with very Httle mathematical help, we find ourselves in a 

 position to deal with some simple examples of organic forms. 



When we melt a stick of sealing-wax in the flame, surface-tension 

 (which was ineffectively present in the solid but finds play in the 

 now fluid mass) rounds off its sharp edges into curves, so striving 

 tbwards a surface of minimal area ; and in like manner, by merely 

 melting the tip of a thin rod of glass, Hooke made the little spherical 

 beads which served him for a microscope*. When any drop of 

 protoplasm, either over all its surface or at some free end, as at the 

 extremity of the pseudopodium of an amoeba, is seen hkewise to 

 "round itself off," that is not an effect of "vital contractility," but, 

 as Hofmeister shewed so long ago as 1867, a simple consequence of 

 surface-tension; and almost immediately afterwards Engelmannf 

 argued on the same lines, that the forces which cause the contraction 

 of protoplasm in general may "be just the same as those which tend 

 to make every non-spherical drop of fluid become spherical." We 

 are not concerned here with the many theories and speculations 

 which would connect the phenomena of surface-tension with con- 

 tractility, muscular movement, or other special 'physiological func- 



* Similarly, Sir David Brewster and others made powerful lenses by simply 

 dropping small drops of Canada balsam, castor oil, or other strongly refractive 

 liquids, on to a glass plate: On New Philosophical Instruments (Description of a 

 new fluid microscope), Edinburgh, 1813, p. 413. See also Hooke's Micrographia, 

 1665; and Adam's Essay on the Microscope, 1798, p. 8: "No person has carried 

 the use of these globules so far as Father Torre of Naples, etc." Leeuwenhoek. 

 on the other hand, ground his lenses with exquisite skill. 



t Beitrage zur Physiologie des Protoplasma, Pfluger's Archlv, ii, p. 307, 1869. 



