XVII] THE COMPAKISON OF RELATED FORMS 1049 



of the forces is simple, yet in the aggregate the system of forces 

 set up by the many 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 (or spheres) would be, from 

 the point of view of the mathematician, an extraordinarily difficult 

 transformation; 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 some- 

 what 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 glass-blower, 

 to which we have already, more than once, referred in passing f. 

 The glass-blower 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 



* Analogous structural differences, especially in the fibrovascular bundles, help 

 to explain the differences between (e.g.) a smooth melon and a cantelupe, or between 

 various elongate, flattened and globular varieties. These breed true to type, and 

 obey, when crossed, the laws of Mendelian inheritance. Cf. E. W. Sinnett, Inherit- 

 ance of fruit-shape in Cucurbita, Botan. Gazette, lxxiv, pp. 95-103, 1922, and other 

 papers. 



t 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. 



