238 ROUND THE YEAR 



The reversed spiral seems therefore to be one of the 

 most perfect contrivances in Nature. I think I see 

 Darwin's admiration of it in many of his expressions. 1 

 Nevertheless Darwin, like Mohl before him and Sachs 

 after him, was aware that the reversal of the spiral is 

 a mechanical necessity. When a band whose ends 

 are not permitted to revolve has a tendency to curl, 

 and consequently to form spirals, it can only form a 

 reversed spiral. This is most easily demonstrated by 

 taking a long ribbon of sheet brass, and winding it 



Fig. 61. Strip of sheet brass, which has wound itself into a reversed spiral. 



into a close coil, as one would wind a tape measure. 

 If one end of such a coil is pulled out, it will take the 

 form of a continuous spiral. Unroll the entire coil, 

 and hold out the ribbon straight and flat. Then 

 gradually relax the pull. The tendency to coil will 

 throw the ribbon into spiral turns, the middle point 

 will revolve, and for every turn from right to left a 

 turn from left to right will appear. The reversed 

 spiral is not a contrivance at all ; it is a mechanical 

 necessity when a band whose ends are not free to 

 revolve is thrown into coils. 



The reversed spiral is often found in Nature where 

 an elastic spring would be quite unnecessary." It is 

 1 Climbing Plants, Chap. IV. 



