LECTUKE XIII. 



ORIGINATIVE FACTORS IN EVOLUTION: 

 VARIATION. 



1. The Central Problem of JBtiology is the Origin of Heritable 

 Variations. 2. Variations Distinguished from Modifica- 

 tions. 3. Discontinuous Variations (or Mutations) and 

 Continuous Variations (or Fluctuations). 4. Problem of 

 the Origin of Variations. 5. Correlation of Variations. 6. 

 Theory of Temporal Variations. 7. Evidences of Definite- 

 ness in Variability. 8. Germ-cells on Implicit Organisms. 



1. The Central Problem of ^Etiology Is the Origin 

 of Heritable Variations. 



WHILE the general idea of evolution is accepted by prac- 

 tically all living naturalists, there is great uncertainty in 

 regard to the factors that have been operative in the process. 

 The uncertainty is partly due to the difficulty of arguing 

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

 millions of years, and partly to the fact that scientific etiol- 

 ogy is still very young, for it may almost be said to date from 

 Darwin's Origin of Species (1859). 



There are two main problems of evolution. The first asks 

 how we are to account for the continual emergence of new 

 things, of changes or variations which make an organism 

 appreciably different from its parents or from the rest of 

 its kin. The second asks what directive factors operate on 

 the variations which arise, determining their elimination 

 or their persistence as the case may be, and working, it may 

 be, towards the familiar but puzzling result the existence of 

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