THE FOUR LAWS OF AGE 225 



their own ; their genetic restriction has not gone so 

 far that all their possibilities of change in the way of 

 differentiation have been fixed ; there is a certain 

 range of possibilities still open to them, and they may 

 turn in one direction or the other. Hence there may 

 be pathological growths of a character not normally 

 present in the body. It seems to me, so far as my 

 knowledge of this subject enables me to judge, to be 

 true that all such pathological growths depend upon 

 the presence of comparatively young and undifferen- 

 tiated cells being turned into a new direction. The 

 problem of normal development and of abnormal 

 structure is one and the same. Both the embryo- 

 logist and the anatomist, on the one hand, and the 

 pathologist and the clinician on the other, deal ever 

 with these questions of differentiation, and practically 

 with no others. All that occurs in the body is the 

 result of various differentiations, and whether we call 

 the state of that body normal or pathological matters 

 little ; still the cause of it is the differentiation of the 

 parts. 



The second of the collateral topics which I should 

 like briefly to allude to is another branch of the study 

 of senescence. The fact was first emphasised by the 

 late Professor Alpheus Hyatt that in many animals 

 there exist parts formed in an early stage and there- 

 after never lost. The chambered nautilus is an animal 

 of this kind. The innermost chamber represents the 

 youngest shell of the nautilus, and as its age increases, 

 it forms a new chamber in its shell, and so yet more 

 is 



