16 
PROOFS OF EVOLUTION. 
III. 
THE FOUR GREAT FACTORS OF EVOLUTION. 
These are (1) The influence of Environment; 
(2) The increased use or disuse of organs; (3) 
Natural Selection, or Survival of the Fittest; and 
(4) Sexual Selection. The two former are accred¬ 
ited to Lamarck and others, but the two latter are 
wholly Darwinian; and running through all is the 
potent influence of heredity. 
A change in environment effects a corresponding 
change in functional activities, which leads to 
structural modification. Common observation and 
experience show us the effect of the use or disuse 
of an organ or part. This is likewise observable 
in determining the greater or less acuteness and 
activity of all the senses. An organ lying dormant 
during many generations will gradually atrophy, 
and appear as a rudiment, finally vanishing alto¬ 
gether. Fishes and animals dwelling in under¬ 
ground caves into which no light enters, are 
sightless, as they have no use for eyes. They have 
