342 PHYSIOLOGICAL DEVELOPMENT. 



non-ossification is in part apparent only — it is rather that 

 new cartilage is formed as fast as the pre-existing cartilage 

 ossifies ; and there is the further reply that the slowness of 

 the ultimate ossification of this part, is due to its non- 

 vascularity, and to mechanical conditions that are unfavour- 

 able to its acquirement of vascularity. Once more, the de- 

 murrer that in the epiphyses ossification does not begin at 

 the surface but witHin the mass of the cartilage, is met by an 

 explanation parallel to that before given (§ 293, note) of the 

 deep-seated induration produced by an external pressure 

 which, during long intervals, does not intermit completely ; 

 as in a bunion, a node on the instep, and what is called 

 " housemaid's knee." 



Of course it is not meant that this osseous development by 

 direct equilibration, takes place in the individual. Though 

 it is a corollary from the argument that in each individual 

 the process must be furthered and modified by the particular 

 actions to which the particular bones are exposed ; yet the 

 leading traits of structure assumed by the bones are assumed 

 in conformity with the inherited type. This, however, is no 

 difficulty. The type itself is to be regarded as the accumulated 

 result of such modifications, transmitted and increased from 

 generation to generation. The actions above described as 

 taking place in the bone of an individual, must be understood 

 as producing their total effect little by little in the corre- 

 sponding bones of a long series of individuals. Even if but 

 a small modification can be so wrought in the individual, yet 

 if such modification, or a part of it, is inheritable, we may 

 readily understand how, in the course of geologic epochs, the 

 observed structures may arise by the assigned way. 



Here ma} 7- fitly come in a strong confirmation. If we find 

 cases where individual bones, subject in exceptional degrees 

 to the actions described, present in exceptional amounts the 

 modifications attributed to them, we are greatly helped in 

 understanding how there may be produced in the race that 

 aggregate of modifications which the hypothesis implies* 



