164 THE PKINCIPLES OF HEREDITY 



Coast being attacked, and many children perishing. Pre- 

 sumably they will continue to suffer to the end, since when- 

 ever evolution approaches perfection, reversion steps in. It 

 follows that if malaria enfeebles offspring it must enfeeble 

 and ultimately exterminate the race notwithstanding the 

 elimination of the unfit. But there is not an iota of evidence 

 that any race has been enfeebled by experience of any disease 

 even when experience of the disease has extended over 

 hundreds, perhaps thousands, of successive generations. 



278. Again, it has been argued that, though malaria or 

 other causes of enfeeblement may as a rule leave most offspring 

 unaffected, it may yet affect a considerable number of offspring. 

 These, being enfeebled, would be eliminated, and thus the 

 race would still undergo evolution. It is difficult to meet 

 arguments which depend wholly on a series of unproved hypo- 

 theses, but it is obvious that such a process would eliminate 

 from the race a particular type of germ-plasm the type that 

 was capable of being altered in its hereditary tendencies. 

 Natural Selection would therefore render the germ-plasm very 

 stable. We have maintained nothing more than that the 

 germ-plasm is stable, and that its stability has been evolved 

 and is still being maintained by Natural Selection. 



279. All evolution is a process of adaptation to the environ- 

 ment. The more complex the environment, the more complex 

 must be the species that dwells therein. Its structures and 

 faculties must be numerous and efficient in proportion to the 

 diversity and magnitude of the contingencies it has to meet. 

 For this reason a ' ' higher " animal is more complex than a 

 " lower." If the environment grows more complex the species 

 must follow suit or suffer extinction. New structures and 

 faculties must be evolved to meet new dangers, or old struc- 

 tures and faculties must be rendered more efficient to meet 

 greater dangers. If the environment grows less complex the 

 species must still follow suit. It must become less complex, or, 

 overburdened with useless structures and faculties, it will no 

 longer be adapted to the conditions of its existence. We 

 know that both these reactions occur in nature. Every 

 species undergoes progressive evolution under adverse condi- 

 tions ; that is, when the complexity of the environment 

 increases. Every species undergoes regressive evolution (i.e. 

 degeneration) under beneficial conditions ; that is, when the 

 complexity of the environment decreases. 1 But all evolution, 



1 The environment is never entirely beneficial. Under every known 

 condition there are always some unfit who are eliminated. Thus civil- 

 ized men, as a rule, have plenty of food, and among them the weak are 



