CHAPTER VIII 
THE FACTORS OF EVOLUTION 
Variation along definite lines and Natural Selection are un- 
doubtedly important factors of evolution—Whether or not 
sexual selection is a factor we are not yet in a position to decide 
—Modus operandi of Natural Selection—Correlation an im- 
portant factor—Examples of correlation—Correlation is a 
subject that requires close study—Isolation a factor in evolu- 
tion — Discriminate isolation—Indiscriminate isolation — Is 
the latter a factor?—Romanes’ views—Criticism of these— 
Indiscriminate isolation shown to be a factor—Summary of 
the methods in which new species arise—Natural Selection 
does not make species—It merely decides which of certain 
ready-made forms shall survive—Natural Selection compared 
to a competitive examination and to a medical board—We 
are yet in darkness as to the fundamental causes of the 
Origin of Species—In experiment and observation rather 
than speculation lies the hope of discovering the nature of 
these causes. 
E have so far considered three factors 
of evolution. The first of these is 
the tendency of organisms to vary 
along definite lines. This is a most 
important factor, because, unless variation occurs 
in any given direction, there can be no evolution 
in that direction. Variations are the materials 
upon which the other factors, or causes, of evolu- 
tion work. The second great factor is natural 
selection. Natural selection may be compared 
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