$6 



THE CAUSATION OF DISEASE. 



changing environment, while other characters, which have 

 remained structurally active during yet longer periods, have, 

 by their tremendous repetition, become so fixedly organized in 

 the great vital rhythm that they even now have a transient 

 existence in the embryo. The accompanying diagram is in- 

 tended to represent the mode in which characters are acquired 

 and dropped again in the progress of organic evolution, also 

 the ancestral age of different characters : — 



Fig. 8. 



Characters acquired during the 

 post -anthropoidal stage. 



Characters acquired during the 



anthropoidal stage. 

 Do. during primitive mammalian 



stage. 

 Do. during amphibian stage. 

 Do. during pre-vertebral and 



piscian stage. 



Amphibian. 



Primitive mammal. 



Anthropoid. 



Diagram representing the Evolution of Man. 



Evolution starts at x. The vertical lines, A, B, c, D, repre- 

 sent the developmental processes of man's ancestry at the 

 following stages : — A, the primitive fish stage ; B, the amphi- 

 bian stage ; c, the stage of the primitive mammal ; D, the 

 anthropoidal stage. The vertical line, E, represents the develop- 

 mental processes of man himself. In all these cases the embryo- 

 logical processes are supposed to start from the centre of the 

 vertical lines and to spread outwards. The horizontal shaded 

 portions indicate the ancestral age of man's several structural 

 characteristics. Thus, fixing the mind upon the vertical line, 

 E, and remembering that development is supposed to start from 

 the centre, we find the most early structural manifestations 

 of the embryo to be of oldest date. These characters were 

 acquired during the invertebrate and piscian stage of man's 



