72 EVOLUTION AND NATURAL THEOLOGY. 



May we not legitimately infer that the vital 

 energy lias limits to its action ; that it works in 

 grooves, and that the deeper channels tend to 

 attract to themselves the energy which would 

 otherwise flow in the shallower ones ? * The 

 necessary result is to increase the total amount 

 of differentiation or division of labour, which 

 attains its maximum physically, in the most 

 highly developed animals, and mentally in the 

 most highly civilized communities. 



The life of the species corresponds, on a larger 

 scale, to that of the individual, and the same 

 laws of use and disuse will apply to both. 

 Generally speaking, like produces like through- 

 out Nature ; but there is at the same time a vast 

 number of differences of more or less magnitude 

 between individuals. The varying constitution 

 of the parents is doubtless one cause of differ- 

 ence in their offspring ; but that it is not the 

 only cause is obvious, if only from the fact that 

 twins, though sometimes resembling each other 

 more than other children of the same parents, 



* On this principle, Darwin's " Law of Correlation of Growth " ; Her- 

 bert Spencer's " Lines of Least Eesistance," and the " Definite Lines of 

 Development," advocated by Asa Gray, Mivart, and others, will admit 

 of a common explanation. 



