598 THE SPIRAL SHELLS [ch. 



This ratio of 1/(1 + q) is easily shewn to be the constant ratio 

 running through the whole series^ from chamber to chamber; 

 and if this ratio of volumes be constant, so also are the ratios 

 of corresponding surfaces, and of corresponding linear dimensions, 

 provided always that the successive increments, or successive 

 chambers, are similar in form. 



We have still to discuss the similarity of form and the symmetry 

 of position which characterise the successive chambers, and which, 

 together with the law of continued proportionality of size, are the 

 distinctive characters and the indispensable conditions of a series 

 of "gnomons." 



The minute size of the foraminiferal shell or at least of each 

 successive increment thereof, taken in connection with the fluid 



Fig. 314. Orbulinn universa, d'Orb. 



or semi-fluid nature of the protoplasmic substance, is enough to 

 suggest that the molecular forces, and especially the force of 

 surface-tension, must exercise a controlling influence over the form 

 of the whole structure ; and this suggestion, or behef, is already 

 implied in our statement that each successive increment of growing 

 protoplasm constitutes a separate drop. These "drops," partially 

 concealed by their successors, but still shewing in part their 

 rounded outlines, are easily recognisable in the various fora- 

 miniferal shells which are illustrated in this chapter. 



The accompanying figure represents, to begin with, the spherical 

 shell characteristic of the common, floating, oceanic Orbuhna. 

 In the specimen illustrated, a second chamber, superadded to the 



