INTERPRETATION AND QUALIFICATION. 475 



things equal, imply that it takes less effort. The creature's 

 sensations will ever prompt desistance from the more laborious 

 course; and hence a congenital habit is not likely to be 

 diverged from unless an economy of force is achieved by the 

 divergence. Assuming, then, that the new method has no 

 advantage over the old in directly diminishing the chances 

 of death, the establishment of it, and of the structural 

 complications involved, nevertheless implies a physiological 

 gain. Suppose, again, that an animal takes to some 

 abundant food previously refused by its kind. It is likely to 

 persist only if that the comparative ease in obtaining this 

 food, more than compensates for any want of adaptation to its 

 digestive organs ; so that superposed modifications of the 

 digestive organs are likely to arise only when an averao-e 

 economy results. What now must be the influence 



on the creature's system as a whole ? Diminished expenditure 

 in any direction, or increased nutrition however effected, 

 will leave a greater surplus of materials. The animal will be 

 physiologically richer. Part of its augmented wealth will go 

 towards its own greater individuation — its size, or its strength, 

 or both, will increase; while another part will go towards 

 more active genesis. Just as a state of plethora directly 

 produced enhances fertility; so will such a state indirectly 

 produced. 



In another way, the same thing must result from those 

 additions to bulk or complexity or activity that are due to 

 survival of the fittest. Any change which prolongs individual 

 life, will, other things remaining the same, further the pro- 

 duction of offspring. Even when it is not, like the foregoing, 

 a means of economizing the forces of the individual, still, if it 

 increases the chances of escaping destruction, it increases the 

 chances of leaving posterity. Any further degree of evolution, 

 therefore, will be so established only where the cost of it is 

 more than repaid; part of the gain being shown in the 

 lengthened life of the individual, and part in the greater 

 production of other individuals. 



