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. 



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 in a position to decide Modus operandi 

 of Natural Selection Correlation an important factor Examples 

 of correlation Correlation is a subject that requires close study 

 Isolation 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 



INDEX ...... 389 



xvi 



