230 EXPERIMENTAL EVOLUTION LECT. 



" Secondly. Use and disuse of organs reacting on 

 growth-force, and producing change in form, structure, 

 and relative size of parts, and such change inherited 

 and integrated through successive generations. 



" Thirdly. Natural selection among individuals of 

 a varying progeny, of those most in accord with an 

 ever-changing environment, or, as it has been other- 

 wise called, survival of -the fittest in each successive 

 generation. 



" Fourthly. Sexual selection. The selection of females 

 among varying male individuals, all competing for the 

 possession of the strongest or the most attractive. 

 Among mammals the selection is mainly of the 

 strongest as decided by battle ; among birds, of the 

 most attractive, as determined by splendour of colour 

 or beauty of song. 



" Fifthly. Physiological selection, or selection of those 

 varieties, the individuals of which are fertile among 

 themselves, but sterile or less fertile with other varieties 

 and with the parent stock. This has also been called 

 segregate fecundity by Gulick, and homogamy by 

 Romanes." 



These five factors are not all recognised by the same 

 group of evolutionists. The two first factors are 

 Lamarckian ; the third and fourth are Darwinian ; the 

 two first are supposed to operate during individual 

 life ; the third and fourth operate on the offspring, 



