The Making of Species 
—Neo-Lamarckian explanation of sexual dimorphism stated and 
criticised—Some features of sexual dimorphism—Dissimilarity 
of the sexes probably arises as a sudden mutation—The four 
kinds of mutations—Sexual dimorphism having shown itself, 
Natural Selection determines whether or not the organisms 
which display it shall survive. 
PAGE 
CHAPTER VIII 
THe Factors oF EVOLUTION : : 345 
Variation along definite lines and Natural Selection are undoubtedly 
important factors of evolution—Whether or not sexual selection 
is a factor we are not yet ina position to decide—Modus operandi 
of Natural Selection—Correlation an important factor—Examples 
of correlation—Correlation is a subject that requires close study 
—lIsolation a factor in evolution—Discriminate isolation—Indis- 
criminate 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 com- 
pared 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, , 
INDEX ‘ : . : ‘ - 389 
xvi 
