210 DEVELOPMENT AND HEREDITY. 



as its associations become weaker and as its 

 reactions are less practised. Growth, therefore, 

 becomes more sluggish. The internal arrangement 

 of forces is not so quickly and easily changed; the 

 external forces remain unchanged, and thus the 

 successive combinations of the two sets of forces 

 change less and less, until finally growth and 

 development practically cease. 



If, at any intermediate stage in this course 

 of development, the particular combinations of forces 

 causing growth of that stage should be changed, 

 it follows that the growth itself at that stage would 

 be changed. We have reason to believe that the 

 manner of growth for some particular period of 

 development may be secondarily changed without 

 radically affecting either the preceding or suc- 

 ceeding growth. As an example of this, may be 

 mentioned the embryonic organs and embryonic 

 modifications which adapt the embryo to undergo 

 a partial development in the body of the parent, 

 and allow it to receive nutriment from the parent, 

 e.g. the placenta. According to the generally 

 accepted theory these modifications have gradually 

 developed upon a form of embryo that was originally 

 adapted to perform its whole development outside 

 the body of the parent. Such modifications can- 

 not have arisen by the backward transference of 



