XVII] THE COMPAEISON OF RELATED FORMS 737 



teeth of the comb is exceedingly complex, and its complexity is 

 revealed in the complicated "diagram of forces" which constitutes 

 the pattern. 



To take another and still more instructive illustration. To 

 turn one circle (or sphere) into two circles would be, from the point 

 of view of the mathematician, an extraordinarily difficult trans- 

 formation ; but, physically speaking, its achievement may be 

 extremely simple. The little round gourd grows naturally, by 

 its symmetrical forces of expansive growth, into a big, round, or 

 somewhat oval pumpkin or melon. But the Moorish husbandman 

 ties a rag round its middle, and the same forces of growth, unaltered 

 save for the presence of this trammel, now expand the globular 

 structure into two superposed and connected globes. And 

 again, by varying the position of the encircling band, or by 

 applying several such ligatures instead of one, a great variety of 

 artificial forms of "gourd" may be, and actually are, produced. 

 It is clear, I think, that we may account for many ordinary 

 biological processes of development or transformation of form by 

 the existence of trammels or lines of constraint, which limit and 

 determine the action of the expansive forces of growth that would 

 otherwise be uniform and symmetrical. This case has a close 

 parallel in the operations of the glassblower, to which we have 

 already, more than once, referred in passing*. The glassblower 

 starts his operations with a tube, which he first closes at one end 

 so as to form a hollow vesicle, within which his blast of air exercises 

 a uniform pressure on all sides ; but the spherical conformation 

 which this uniform expansive force would naturally tend to 

 produce is modified into all kinds of forms by the trammels or 

 resistances set up as the workman lets one part or another of his 

 bubble be unequally heated or cooled. It was Oliver Wendell 

 Holmes who first shewed this curious parallel between the 

 operations of the glassblower and those of Nature, when she starts, 

 as she so often does, with a simple tubej. The alimentary canal, 



* Where gourds are common, the glass-blower is still apt to take them for a 

 prototype, as the prehistoric potter also did. For instance, a tall, annulated 

 Florence oil-flask is an exact but no longer a conscious imitation of a gourd which 

 has been converted into a bottle in the manner described. 



t Cf. Elsie Venner, chap. ii. 



T. G. 47 



