TRADITION AND HEREDITY 459 



that disease, which is chiefly noticeable within the third period, 

 upon the whole then takes a lethal form from which the patient 

 either dies or recovers more or less completely. In tropical regions, 

 however, there are a number of diseases of the non-lethal chronic 

 type, such as the hook-worm disease for instance, which affect 

 a large percentage of the population and sap the energy — mental 

 and physical — of those attacked. The prevalence of any such 

 disease must act in an important fashion as a drag upon progress, 

 and the spreading of such a disease into a country previously 

 unaffected may quite conceivably put a stop to progress, and even 

 bring about a condition of decadence. The decline of Greece has 

 been attributed, for instance, to the introduction of malaria. In 

 answering the question, therefore, why certain areas have lagged 

 behind others, we must not forget the influence of chronic disease 

 when it affects a large proportion of the population, and it is 

 a curious fact that in general the working of this factor has been 

 in the same direction as that of other factors which influenced 

 tradition. Where, that is to say, there has been a low degree of 

 fertility in the environment and little stimulus through contact, 

 there also upon the whole the deleterious effect of disease has been 

 most pronounced. 



It may further be noticed that the direct effect of the environ- 

 ment has a bearing upon the moulding of tradition. The apathy 

 and listlessness, for example, resulting from the prevalence of 

 a disease like malaria form an element in the environment and 

 give an unfavourable turn to the development of tradition. 



14. Something has been said in the last two chapters as to the 

 nature and formation of tradition, and in this chapter as to its 

 influence in determining the main outlines of the course of history 

 in the chief geographical divisions of the world. The part played 

 by tradition may be best further illustrated by a reference to two 

 problems to which allusion has already been made— namely, the 

 cyclical course of civilization and modern differential fertility. 

 When we speak of the cyclical course of civilization we have in 

 mind the alternations of periods exhibiting vigour and energy 

 with periods exhibiting apathy. As within the same culture we 

 see progress along one line and within one atmosphere. At times, 

 more especially when the degree of skill is relatively low, there may 

 be scarcely any progress and the condition is one of stagnation. 

 But under such circumstances any turning back is rare. On the 



