THE UNIQUENESS OF THE INDIVIDUAL 



of articular surfaces and the patterns of bony trabeculae. With 

 this quahfication, the mode of development of joints is closely 

 comparable to that of flexure lines and thickened soles. We are 

 born with working joints, cartilage lined, encapsulated, lubri- 

 cated with synovial fluid, and with their apposed surfaces 

 having just that complementarity of structure which might be 

 expected to arise from the mechanical exactions of ordinary 

 use. Foetal movements have only a small part to play in 

 fashioning the final structure, for joints develop from primor- 

 dia cultivated ^7l vitro or transplanted to the chorio-allantoic 

 membrane — positions where no movement can occur. In spite 

 of that, functional cartilage-lined and encapsulated ectopic 

 joints can be formed in an individuaPs later lifetime if by 

 accident (or orthopaedic artifice) two mobile bony surfaces are 

 apposed to each other, as in an unhealed fracture (see Le Gros 

 Clark, 1952). Here too then, it appears, the mechanisms of 

 morphogenesis exist in duplicate, and what could be formed 

 by use is in fact formed by pre-emptive diff'erentiation. 



In the foregoing account I have deliberately confined myself 

 to familiar everyday examples of pre-emptive diff'erentiation in 

 metazoa. (Micro-organisms come later.) The more esoteric 

 examples collated by Wood Jones (1943) ^ appear to me to 

 introduce no distinction of principle, and an explanation valid 

 for the one set should be valid for the other. Each represents a 

 character diff'erence of developmental origin that could also 

 have arisen as a direct adaptive response to difference of use 

 within an individuaPs own lifetime. 



All such adaptations are open to a Lamarckian interpreta- 

 tion of their origin. All that remains to establish a strong /?m«a 

 facie case is evidence that acquired character diff'erences can 



1 For example the squatting facets between tibia and ankle-bone in 

 Panjabi (but see Medawar, 1952) and the callosities on the 'knees' of the 

 African wart-hog; to which add Kukenthal's strange story of the dugong's 

 teeth, as it has been recounted by de Beer (1951). 



88 



