228 GENERAL EVOLUTION. 



ercise of an organ determines nutritive material to it, and the 

 nervous or other influence which does this, equally determines 

 nutritive material to localities in the body to which an effort to 

 move is directed, whether an executive organ exist there or not. 

 The habit of effort or use determining the nutritive habit must be 

 inherited, and result in the growing young, in additional struct- 

 ure. Change of structure, denied to the adult on account of its 

 fixity, will be realized in the growing or plastic condition of foetal 

 or infant life. The two considerations here brought forward lead 

 me to think that the cause of acceleration, in many adaptive 

 structures, is environment alone, or environment producing move- 

 ments, which in turn modify structure. The character of the 

 stimulus in the successive grades of life may be expressed by the 

 following table, passing from the lowest to the highest : 



1. Passive or motionless beings : 



by climate and food only. 



2. Movable beings : 



by climate, food and motion. 

 By motion either : 

 a, unconscious, or * 

 aa, conscious, which is, 



h, reflex, or 

 M, directed by desire without ratiocination, or 

 Mb, by desire directed by reason. 



The only general rules as to the direct influence of motion on 

 structure which can be laid down at present are two, viz., That 

 density of tissue is in direct ratio to pressure, up to a certain 

 point ; t and that excess of growth-force, in a limited space, pro- 

 duces complications of the surfaces stimulated. \ These and other 

 laws, yet unknown, have probably led the changes expressed by 

 evolution, while many others have followed the disturbance of 

 equilibrium which they have produced. 



I here allude incidentally to the question of transmission or 

 inheritance. It has been maintained above that the bathmic force 

 of each species is different from that of all other sj^ecies. This 

 force is characteristic of some unit of organization of living be- 



* Movements coming under this head are often called reflex, 

 •f- See "Penn. Monthly," 1872; this work Art. L 

 X " Method of Creation," Philadelphia, 1871. 



