316 SIMPLE AND COMPOUND EVOLVTION. 



fully understood apart from the rest and no one of which 

 can be studied simultaneously with the rest. Already we 

 have seen that during Evolution two great classes of changes 

 are going on together; and we shall presently see that the 

 second of these great classes is re-divisible. Entangled with 

 one another as all these changes are, explanation of any one 

 class or order involves direct or indirect reference to others 

 not yet explained. We have nothing for it but to make the 

 best practicable compromise. 



It will be most convenient to devote the next chapter to 

 a detailed account of Evolution under its primary aspect; 

 tacitly recognizing its secondary aspects only so far as the 

 exposition necessitates. 



The succeeding two chapters, occupied exclusively with 

 the secondary re-distributions, will make no reference to the 

 primary re-distribution beyond that which is unavoidable: 

 each being also limited to one particular trait of the sec- 

 ondary re-distributions. 



In a further chapter will be treated a third, and still 

 more distinct, character of the secondary re-distributions. 



