688 ON FORM AND MECHANICAL EFFICIENCY [ch. 



whose physical cause is as obscure as its final cause or end is, 

 apparently, manifest. 



Partly associated with the same phenomenon, and partly to 

 be looked upon (meanwhile at least) as a fact apart, is the very 

 important physiological truth that a condition of strain, the 

 result of a stress, is a direct stimulus to growth itself. This indeed 

 is no less than one of the cardinal facts of theoretical biology. 

 The soles of our boots wear thin, but the soles of our feet grow- 

 thick, the more we walk upon them : for it would seem that the 

 living cells are "stimulated" by pressure, or by what we call 

 " exercise," to increase and multiply. The surgeon knows, when 

 he bandages a broken hmb, that his bandage is doing something 

 more than merely keeping the parts together : and that the even, 

 constant pressure which he skilfully apphes is a direct encourage- 

 ment of growth and an active agent in the process of repair. In the 

 classical experiments of Sedillot*, the greater part of the shaft of the 

 tibia was excised in some young puppies, leaving the whole weight 

 of the body to rest upon the fibula. The latter bone is normally 

 about one-fifth or sixth of the diameter of the tibia; but under 

 the new conditions, and under the "stimulus" of the increased 

 load, it grew till it was as thick or even thicker than the normal 

 bulk of the larger bone. Among plant tissues this phenomenon 

 is very apparent, and in a somewhat remarkable way ; for a strain 

 caused by a constant or increasing weight (such as that in the 

 stalk of a pear while the pear is growing and ripening) produces 

 a very marked increase of strength without any necessary increase 

 of bulk, but rather by some histological, or molecular, alteration 

 of the tissues. Hegler, and also Pfeffer, have investigated this 

 subject, by loading the young shoolj of a plant nearly to its 

 breaking point, and then redetermining the breaking-strength 

 after a few days. Some young shoots of the sunflower were found 

 to break with a strain of 160 gms. ; but when loaded with 150 gms., 

 and retested after two days, they were able to support 250 gms. ; 

 and being again loaded Avith something short of this, by next day 

 they sustained 300 gms., and a few days later even 400 gms. 



* Sedillot, De I'influence des fonctions sur la structure et la forme des organes; 

 C. R. Lix, p. 539, 1864; cf. lx, p. 97, 1865, Lxvm. p. 1444. 1869. 



