XVI] ON STRESS AND STRAIN 987 



of increasing strength, following mechanically on a rearrangement 

 of molecules, as the original isotropic condition is transmuted more 

 and more into molecular asymmetry or anisotropy; and the rope 

 apparently "adapts itself" to the increased strain which it is called 

 on to bear, all after a fashion which at least suggests a parallel to 

 the increasing strength of the stretched and weighted fibre in the 

 plant. For increase of strength by rearrangement of the particles 

 we have already a rough illustration in our lock of wool or hank 

 of tow. The tow will carry but little weight while its fibres are 

 tangled and awry: but as soon as we have carded or "hatchelled" 

 it out, and brought all its long fibres parallel and side by side, we 

 make of it a strong and useful cord*. 



But the lessons which we learn from dough and treacle are 

 nowadays plain enough in steel and iron, and become immensely 

 more important in these. For here again plasticity is associated 

 with a certain capacity for structural rearrangement, and increased 

 strength again results therefrom. Elaborate processess of rolling, 

 drawing, bending, hammering, and so on, are regularly employed to 

 toughen and strengthen the material. The "mechanical structure" 

 of solids has become an important subject. And when the engineer 

 talks of repeated loading, of elastic fatigue, of hysteresis, and other 

 phenomena associated with plasticity and strain, the physiological 

 analogues of these physical phenomena are perhaps not far away. 



In some such ways as these, then, it would seem that we may 

 coordinate, or hope to coordinate, the phenomenon of growth with 

 certain of the beautiful structural phenomena which present them- 

 selves to our eyes as "provisions," or mechanical adaptations f, for 

 the display of strength where strength is most required. That is 

 to say the origin, or causation, of the phenomenon would seem to 

 lie partly in the tendency of growth to be accelerated under strain : 

 and partly in the automatic effect of shearing strain, by which it 

 tends to displace parts which grow obliquely to the direct lines of 

 tension and of pressure, while leaving those in place which happen 

 to he parallel or perpendicular to those lines : an automatic effect 



* Cf. Sir Charles Bell's Animal Mechanics, chap, v, "Of the tendons compared 

 with cordage." 



f So P. Enriques (op. cit. supra, p. 5), writing on the economy of material in the 

 construction of a bone, admits that "una certa impronta di teleologismo qua e la 

 e rimasta, mio malgrado, in questo scritto." 



