LAMARCKIAN AND DARWINIAN DOCTRINES 67 



animal as growing strong and active through contending with 

 rivals, escaping from enemies, or pursuing prey ; or else as perishing 

 in the fullness of his vigour. They imagined the plant as over- 

 whelmed by the storm, or as emerging more hardy and firmly 

 rooted from the struggle. In brief they conceived the same agent 

 as eliminating the weak and as strengthening the strong. They 

 had no difficulty, therefore, in believing that a hypothesis which 

 attributed evolution to the transmission of acquirements was com- 

 patible with one which attributed it to the survival of the fittest- 

 They had thought of no facts by which they could test this 

 opinion. Consequently for many years scientific men were 

 adherents of both doctrines, as a few still are. 



105. But agencies which, while strengthening the survivors, 

 destroy the weak by violent deaths, are not the only causes of 

 elimination. Some agencies, for example many human diseases, 

 both destroy the weak and weaken the strong. Apparently they 

 are never other than harmful, at any rate they are never beneficial. 

 Almost every individual of many human races is exposed to one 

 or more agencies of this sort. If, then, the Lamarckian doctrine is 

 true, such agencies should be causes of continuous racial deteriora- 

 tion. On the other hand, if that doctrine is false and the Dar- 

 winian doctrine true, if the acquirements of the parent end with 

 the parent and are not inherited by the child, then such agencies 

 should, through the raising of the racial average in every generation 

 by the survival of the fittest, be causes of protective evolution. 

 Here the two theories are in conflict ; we see plainly that they 

 are incompatible ; if the one gives a true account of racial 

 change, the other does not ; for it is impossible that a race can 

 both deteriorate and improve in the same characters at the 

 same time. Therefore we are able to test them by means of 

 crucial instances; that is, we are able to appeal to reality for 

 guidance. The fact that every race (e.g. English and Negro) 

 that has been exposed to any disease (e.g. tuberculosis or malaria) 

 has undergone no discoverable deterioration, but has become 

 resistant to the disease, has evolved against it, precisely in pro- 

 portion to the length and severity of its past affliction, is decisive 

 evidence against the Lamarckian and in favour of the Darwinian 

 doctrine. 



106. It is true that some human diseases (e.g. measles) leave 

 the individual who recovers from them with an increased power of 

 resisting subsequent attacks, and that generations that have 

 suffered from these maladies are on that account proportionately 



