PROBLEMS OF BIOLOGY 319 



to the preceding solidified wall : so giving a simple and 

 adequate physical explanation of what Sachs had stated as 

 an empirical morphological rule. And Berthold further 

 showed how, when the cell-partition was curved, its precise 

 curvature as well as its position was in accordance with 

 physical law. 



There are a vast number of other things that we can 

 satisfactorily explain on the same principle and by the same 

 laws. The beautiful catenary curve of the edge of the pseudo- 

 podium, as it creeps up its axial rod in a Heliozoan or a Radio- 

 larian, the hexagonal mesh of bubbles or vacuoles on the 

 surface of the same creatures, the form of the little groove 

 that runs round the waist of a Peridinian, even (as I believe) 

 the existence, form and undulatory movements of the un- 

 dulatory membrane of a Trypanosome, or of that around the 

 tail of the spermatozoon of a newt : every one of these, I 

 declare, is a case where the resultant form can be well explained 

 by, and cannot possibly be understood without, the pheno- 

 mena of surface-tension. Indeed in many of the simpler 

 cases, the facts are so well explained by surface-tension that 

 it is difficult to find place for a conflicting, much less an 

 over-riding force. 



I believe, for my own part, that even the beautiful and 

 varied forms of the foraminifera may be ascribed to the same 

 cause, but here the problem is a little more complex, by 

 reason of the successive consolidations of the shell. Suppose 

 the first cell or chamber to be formed, assuming its globular 

 shape in obedience to our law, and then to secrete its cal- 

 careous envelope. The new growing bud of protoplasm, 

 accumulating outside the shell, will, in strict accordance with 

 the surface-tensions concerned, either fail to * wet ' or to 

 adhere to the first-formed shell, and will so detach itself as a 

 unicellular individual (Orbulind) ; or else it will flow over a 

 less or greater part of the original shell, until its free surface 

 meets it at the required angle of equilibrium. Then accord- 



