76 FOOT-NOTES TO EVOLUTION. 
Its influence is shown in the persistence of type, in the 
existence of broad homologies among living forms, in 
the possibility of natural systems of classification in 
any group, in the retention of vestigial organs, in the 
early development and subsequent obliteration of out- 
worn structures once useful to the race or type. 
The physical basis of heredity has been in recent 
years the subject of many elaborate investigations, The 
complete homology of the germ cell with the one-celled 
animals, or protozoa, is now generally recognised, and 
there is large reason to believe that in the bands and 
loops of the nucleus of the germ cell is found the visible 
vehicle by which hereditary tendencies are transmitted. 
II. Lrritability.—All living beings are affected by 
their environment. Living matter must always respond 
in some degree to every external stimulus. All living 
beings are moved by or react from every phase of their 
surroundings. The nervous system and its associated 
sense organs are directly related to the conditions of 
life. They are concessions made to the environment. 
The power of motion, whatever it may be, requires the 
guidance obtained from the impressions made by ex- 
ternal things. In all animals this knowledge, whatever 
its degree of completeness, tends to work itself out in 
action, In plants the same thing is in some degree 
true. The essential difference is that, having no power 
of locomotion, the plant is without a general sensorium. 
The parts that move—growing rootlets, tips of branches, 
and the like—have sensibility and power of motion in 
the same series of cells. The animal, a colony of cells 
which move as a whole, has a specialized nervous sys- 
tem which guides the whole. 
Asa rule, the environment does not act directly on 
the individual. Its influence is felt chiefly in modifying 
its action, in increasing, diminishing, or changing its 
