PATHOLOGICAL STATES IN EVOLUTION. 243 



be stated in more general terms and applied to every tissue and 

 organ, provided we add 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 mor- 

 phology. 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 trabeculee which suppoi't 

 the bone in every direction where extra stresses are applied by 

 normal function. The important point to note is the fact that 

 femoral developement 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 "spontaneous" variation, unless 

 we attribute to "spontaneous" a meaning which Darwin never 

 gave it, seeing that he denied knowing how variation arose. All 

 the variations were definite responses, and it is easy to infer that 

 before response became rapid and easy every kind of disaster 

 and disablement must have occurred to those subjected to re- 

 action-provoking stresses. The very process of adaptation (and 

 on these lines "adaptation" is no longer a mystic word) implies 

 long periods of disordered function and poor structural response 

 even in those who survived after repair. But now bone is so 

 plastic and fluent that when it is grafted the osteoblasts and 

 osteoclasts shape it according to the form of the main bone of 

 wliich it becomes a part. 



When we speak of repair it may be noted that the treatises 

 on this subject are strictly limited in their pui'view. They mostly 

 follow Hunter, a vitally important figure in the history of 

 pathology and indeed of all medical science, who, however, 

 lacked the apparatus of knowledge now at every one's disposal. 

 We learn a great deal about the repair of wounds and fractures : 

 of the functions of the fibroblasts or of the wandeiing cells 

 of the blood-stream, and are told, lately, much of regeneration, 

 but of the evolutionary value of organized exudations we hear 

 nothing. ISTor has it been suggested that it is to this and 

 analogous processes that mvich new structure is due. That this 

 is so is strikingly apparent, as I shall attempt to show, in many 

 organs of a highly specialized type. In no structure, perhaps, is 

 the process so clearly seen as in the mammalian heart, which is 

 a perfect museum of evolutionary failures and dislocations, com- 

 pensated for by an extraordinary complication of patched-up 

 tissues and organized exudations in which, perhaps, one tissue 

 takes on the functions of another and some evolutionary rem- 

 nants long survive without function. I was, indeed, first led to 

 take this genei-al view of the variational value of pathological 

 conditions by observing that the heart, when laid open from any 



