REPAIR IN EVOLUTION 71 



of them, show it. It is, in fact, a universal principle. As 

 beavers patch up a dam when it yields or threatens to give 

 way, so tissues, organs, and societies react to threatened 

 disaster. In no tissue is this clearer than in bone. It 

 is true that Wolff's law only deals directly with mechanical 

 stresses, since it runs: " Every change in the form and 

 position of the bones or their function is accompanied 

 by certain definite changes in their internal architecture, 

 and by equally definite secondary alterations of their 

 external conformation in accordance with mathematical 

 law " ; but I hope to show reasons for concluding that 

 such a law may be stated in more general terms, and 

 applied to every tissue and organ, provided we add, as 

 suggested before, that the more complex the tissue or the 

 organ the greater the liability of failure, and that each 

 tissue reacts in a typical way. 



It is unnecessary to go into details of osteogenesis and 

 morphology. It has been recognized by engineers that the 

 head of the femur is formed exactly in accordance with 

 mechanical law. Had any of them been required to design 

 a structure fit for undergoing the stresses borne by the femur 

 in its development and after-life, he would have sketched a 

 figure extremely like it, not only in its general shape, but 

 in the trabecular which support the bone in every direc- 

 tion where extra stresses are applied by normal function. 

 The important point to note is the fact that femoral 

 development follows stress in individual development, 

 from which we must draw the conclusion that it followed 

 stress during evolution, not that its value for complex- 

 function was gradually increased by chance or " spon- 

 taneous " variation, unless we attribute to " spontaneous ' 

 a meaning which Darwin never gave it, seeing that he 

 denied knowing how variation arose. All the variations 



