ENVIKONMENT AMONG MEN 349 



as to the ravages of these parasites. In a public lecture, delivered 

 before the University of Pennsylvania, in 191 5,^ Allan Smith, 

 citing the factors which have caused the Southern States to lag 

 in American progress, mentioned that the hook-worm disease 

 ' stands with malaria as worse than wars and the devastations 

 of battles and worse than all other pathogenic agencies in com- 

 bination'. Through the influence of these diseases ' the men and 

 women of the South, bred from the best colonial stock, offspring 

 of pioneers, with the blood of English gentry and of continental 

 cavaliers in their veins, sank lower and lower in physical degenera- 

 tion and squalor, were derided and denounced as lazy and shiftless 

 and condemned in popular opinion as worthless and a disgrace '. x 



There are many other tropical and sub-tropical diseases of 

 a somewhat similar nature. Among them may be mentioned 

 the * ya^vs ' marked by various eruptions probably due to 

 a spirochaete the guinea-worm, which lives buried in con- 

 nective tissue ; bilharzia, a trematode worm which lives in the 

 bladder and infects nearly 50 per cent, of the inhabitants of 

 Egypt ; and elephantiasis, due to a worm related to the guinea- 

 worm. The manner and degree in which these various diseases 

 affect the nervous tone varies very much. In general the result 

 is debility and a lowering of the nervous tone. So too malaria is 

 a cause of debility, and the main reason for the decadence of Ancient 

 Greece has been sought in this disease. 2 



9. The results of the inquiry so far, though illuminating, are 

 indefinite. This is largely because we are ignorant regarding 

 innate differences. The consideration of the evidence as to 

 identical twins offers a way out of this difficulty. It is known 

 that there are two kinds of twins. There are the so-called identical 

 twins, between whom there is a very close resemblance, and other 

 twins between whom there is no greater resemblance than between 

 any two children of the same parents. It was formerly thought 

 that there was a sharp distinction between the two kinds of 

 twins identical twins arising from a single ovum, which com- 

 pletely separated into two halves during early development, each 

 half giving rise to one child, ordinary twins arising from the 

 simultaneous fertilization of two ova. If this was so, then 

 identical twins would always have approximately the same 



1 Leiper, ' Some Inhabitants of Man ', p. 151 (in Animal Life and Human Progress, 

 edited by Dendy), 8 See Jones, Malaria and Greek History. 



