420 MIND IN EVOLUTION CHAP. 



3. We saw in Chapter III. that all organic life rests on 

 adaptation. The means of maintaining life differ from 

 species to species, and improvement of organisation the 

 method of Orthogenic Evolution, is one means among the 

 rest. It may be shown that this method is not only (as 

 we have assumed) a higher, but also a more efficient 

 method than any other. If we look on the life of a 

 species, its structure and normal functions, as something 

 adapted, I will not say to perfecting, but merely to 

 maintaining itself, it is clear that the efficiency of the 

 adaptation may be measured broadly by the proportion 

 of individuals that it is able to preserve through their 

 normal life span. If the normal life span is difficult to 

 measure, we may take instead the less severe test of 

 maturity, and measure the efficiency of organisation by the 

 number of individuals which it is capable of maturing to 

 the point at which they in turn can propagate the species 

 compared with the number of those that perish by the 

 way. 



Of this proportion we can judge in a rough and ready 

 way by the birth-rate of any given species. For unless 

 the numbers of the species are to increase or decrease in 

 geometrical proportion, the adults must year by year be 

 replaced by, on the average, an equal number of the new 

 generation, neither more nor less. For this purpose, the 

 less efficient the species, the greater the number of young 

 that will be required. Accordingly, unless in any species 

 we have evidence for a steady growth or diminution of 

 numbers, we may fairly take the birth-rate as a rough test 

 of efficiency of organisation. The better the species 

 is adapted to maintain itself, the lower will be the birth- 

 rate, and conversely. 1 



The theory supposes a physical change occurring for physical reasons, 

 and carrying with it a change of behaviour which happens to suit the 

 organism a pure chance which must be repeated every time a new 

 instinct or new variety -of instinct arises. The element of continuity which 

 was the essence of the older evolutionary theory has disappeared and we 

 are left with a succession of leaps, which if they only landed us in 

 varieties of form might be credible, but when they end in complexities of 

 behaviour accurately adjusted to need pass the limit of belief in casual 

 coincidence. 



1 The birth rate for this purpose means the average number of young 



