LECTl'KK XI 1L 



OK1G1NAT1VK FACTORS IN KVOLl'TlOX: 



VARIATION. 



I. Tht Central rroblem of .l^tiotooi/ is the Origin of Heritable 

 }'ariations. '2. \\iriations Distinguished from Modifica- 

 tion*. i>. Discontinuous }'ar;ations (or Mutations} and 

 Continuous }'ariations (or Fluctuations). 4. Problem of 

 the Origin of }'ariations. <">. Correlation of Variations. tf. 

 Theori/ of Temporal \'ariations. 7. Ki'idene-es of Definite- 

 nesf in Variability. S (>\ rin-eells on Implicit Organisms. 



I. The Central rroblem of Etiology If tlie Origin 



of Heritable Variations, 



WHU.K tlu' iiviUM-:il uh':i of evolution is :u\vptiil by prao- 



tic'alh all liviiii:' naturalists, thoiv is iiTeat uncertainty in 



~ > 



regard to the taetors that have been operative in the pi\v. ss. 

 Tlu^ uncertainty is partly due to the difficulty of arirniiii: 

 from a meagre experienee of the present to a past of many 

 millicnis of vears. anol nartlv to the faet that scientific ivtiol- 



t 



OJTV is still verv vounii. for it mav almost be said to date from 



* i *. %' 



Oarwin's Origin of Species ^iSo^). 



There art' two main problems of evolution. The tirst asks 

 how we are to aeeonnt for the continual emergence of new 

 things, of changes or variations whieh make an organism 

 appreeiablv ditYerent from its parents or from the rest of 

 its kin. The seeond asks what direetive faetors operate on 

 tlu> variations whieh arise, determining their elimination 

 or their persistence as the ease may be. and working, it may 

 be. towards the familiar but pu.vliiii: result- -the existence of 



I V 



407 



