INTERPRETATION AND QUALIFICATION. 501 



Each increment of evolution entails a decrement of repro- 

 duction which is not accurately proportionate, but somewhat 

 less than proportionate. The gain in the one direction is not 

 wholly cancelled by a loss in the other direction, but only 

 partially cancelled : leaving a margin of profit to the species. 

 Though augmented power of self-maintenance habitually 

 necessitates diminished power of race-propagation, yet the 

 product of the two factors is greater than before; so that the 

 forces preservative of race become, thereafter, in excess of the 

 forces destructive of race, and the race spreads. We shall 

 soon see why this happens. 



Every advance in evolution implies an economy. That any 

 increase in bulk, or structure, or activity, may become estab- 

 lished, the life of the organism must be to some extent 

 facilitated by the change the cost of self-support must be, 

 on the average, reduced. If the greater complexity, or the 

 larger size, or the more agile movement, entails on the in- 

 dividual an outlay that is not repaid in food more-easily 

 obtained, or danger more-easily escaped; then the individual 

 will be at a relative disadvantage, and its diminished posterity 

 will disappear. If the extra outlay is but just made good 

 by the extra advantage, the modified individual will not sur- 

 vive longer, or leave more descendants, than the unmodified 

 individuals. Consequently, it is only when the expense of 

 greater individuation is out-balanced by a subsequent saving, 

 that it can tend to subserve the preservation of the indi- 

 vidual, and, by implication, the preservation of the race. 

 The vital capital invested in the alteration must bring a 

 more than equivalent return. A few instances 



will show that, whether the- change results from direct 

 equilibration or from indirect equilibration, this must happen. 

 Suppose a creature takes to performing some act in an un- 

 usual way leaps where ordinarily its kindred crawl, eludes 

 pursuit by diving instead of, like others of its kind, by swim- 

 ming along the surface, escapes by doubling instead of by 

 speed. Clearly, perseverance in the modified habit will, other 



