INTRODUCTION 7 



distinctive characteristic of forward evolution is not the 

 elimination of defects, or the acquisition or preservation of 

 advantages, but the parallel development of both defects 

 and advantages, accompanied by a continually increasing 

 output of energy. 



When we inquire what are the most favourable conditions 

 for survival, we find that increased complexity, or advance 

 in evolution, is not one of them. Victims are taken from 

 every grade, and the highest has no immunity. The ex 

 planation appears to be this what is demanded for survival 

 is a very nice adjustment of the whole organism to its sur 

 roundings. When that adjustment is disturbed, either by 

 the excessive development of any single principle in the 

 animal, to the prejudice of the rest of its organism, or by any 

 excessive change in the environment (such, for instance, as 

 the appearance on the scene of a competing species) to 

 which the organism is unable to respond, destruction ensues. 

 What, then, are required for survival are, first, a sufficient 

 but not excessive plasticity, and, secondly, that, when an 

 animal either advances or recedes, the development or the 

 degeneration should be general, and not confined to any 

 one part of its organism. This disturbance of the adjust 

 ment is at least as likely to take place in the higher as in 

 the lower ranks of life. Finally, the changes in the environ 

 ment are usually of a kind which cannot be foreseen. 



This equal liability to extinction, which is common to 

 all grades of evolution, we may express in other terms as 

 the parallel progression of adaptation and misadaptation ; 

 thereby meaning that, so long as a species survives, the 

 dangers to which it is exposed are always counterbalanced, 

 but not more than counterbalanced, by its capacity for 

 resistance. The ratio of gain to loss is constant. Both 

 sides of the equation increase with its development, and 

 diminish with its degeneration, and the gain and loss on 

 either side are about equal throughout. The first ethical 

 deduction from this principle is the following. It is fair 

 to presume, at least provisionally, that what is true of the 

 organism taken as a whole may also be true of each of the 



