ir 252h) 
CHAPTER XI. 
NATURAL SELECTION BY THE STRUGGLE FOR EXIST- 
ENCE. DIVISION OF LABOUR AND PROGRESS. 
Interaction of the Two Organic Formative Causes, Inheritance and Adapta- 
tion.—Natural and Artificial Selection.—Struggle for Existence, or 
Competition for the Necessaries of Life.—Disproportion between the 
Number of Possible or Potential, and the Number of Real or Actual 
Individuals.—Complicated Correlations of all Neighbouring Organisms. 
—Mode of Action in Natural Selection.—Homochromic Selection as the 
Cause of Sympathetic Colourings.—Sexual Selection as the Cause of the 
Secondary Sexual Characters.—Law of Separation or Division of 
Labour (Polymorphism, Differentiation, Divergence of Characters).— 
Transition of Varieties into Species.—Idea of Species.—Hybridism.— 
Law of Progress or Perfectioning (Progressus, Teleosis). 
In order to arrive ata right understanding of Darwinism, 
it is, above all, necessary that the two organic functions 
of Inheritance and Adaptation, which we spoke of in 
our last chapter, should be more closely examined. If we 
do not, on the one hand, examine the purely mechanical 
nature of these two physiological activities, and the various 
action of their different laws, and if, on the other hand, we 
do not consider how complicated the interaction of these 
different laws of Inheritance and Adaptation must be, we 
shall not be able to understand how these two functions, by 
themselves, have been able to produce all the variety of 
