178 A Review of the Modern Doctrine of Evolution. { March, 
I. 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, 
6, reflex, or 
66, directed by desire without ratiocination, or 
bbb, 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 ;? 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 species. This 
force is chgracteristic of some unit of organization of living 
beings ; and this probably consists of several molecules. This 
unit has been termed, by Haeckel, the plastidule. The trans- 
mission of the bathmic force of one generation to another would 
be effected by the transmission of one or more living plastidules; 
and this is probably precisely what is accomplished in reproduc- 
tion. The Dynamic Theory of reproduction I proposed in 1871,* 
and. it has been since adopted by Haeckel under the name of 
perigenesis. I compared the transmission of bathmic force to 
that of the phenomenon of combustion, which is a force conver- 
sion transmitted from substance to substance by contact. The 
recent observations of Hertwig, Bütschli and others, confirm this 
view. The theory of pangenesis, devised to explain the phenom- 
enon of reproduction, is to my mind quite inadequate. 
1 Movements coming under this head are often called reflex. 
2See Penn Monthly, 1872. 
3 « Method of Creation,” Philadelphia, 1871. 
t“ Method of Creation,” 1871. 
