THE PENINSULA OF YUCATAN. 181 



where the prevalent natural growth is of the types which we have defined as bush and 

 jungle. In such regions it is possible for a comparatively inefficient people to get a living by 

 agriculture. The small trees or bushes with a diameter of 5 inches or less can readily be 

 hacked down with almost any kind of heavy knife, while the larger ones can be girdled 

 by cutting off the bark near the base, and will soon die. Pro\dded this is done during 

 the earlier part of the dry season, which is characteristic of all tropical regions where 

 bush or jungle prevails, the bushes and perhaps some of the girdled trees will be dry 

 enough to burn before the rains come again. Hence it is a comparatively simple matter to 

 clear a tract and plant it. If some of the few larger trees of the jungle remain standing, 

 little harm is done. 



In the true forest the case is quite different. In the first place the trees are large, the 

 majority having trunks at least a foot in diameter and many of them much more. More- 

 over, their wood is frequently hard. Hence it is difficult to cut them down. Only people 

 of great energy are capable of doing so on any large scale. If the much easier process of 

 girdling is resorted to, the trees will die in course of time, and it might seem as if even the 

 inefficient people of the tropics could thus clear large areas. Unfortunately another 

 difficulty arises, one which is serious where the trees are actually cut, and much more so 

 where they are girdled. The chmate of the true tropical forests is so uniformly moist 

 that, even when trees have been felled, it takes a long time for them to become dry enough 

 to burn. ^Moreover, while they are drj'ing, new vegetation at once begins to sprout, and 

 by the time the trees are ready to burn the new growth is so large that it prevents the 

 fire from spreading from tree to tree. That this is so is evident from the fact that even in 

 the jungle region the fires which are lighted eveiy year in the spring to burn off the corn- 

 stalks rarely spread to any great distance in the uncut jungle. The speed with which 

 plants grow in the tropics is far more than we commonly realize. One day on the southern 

 edge of the jungle, near the forest but well out of it, my guide remarked that the land 

 over which we were passing had been cultivated 3 years before. Already the bushes were 

 15 feet high. In the heart of the forest the growth is even faster. Hence the very rankness 

 of the growth of vegetation is one of the primary reasons why man has never yet really 

 mastered any considerable area where genuine tropical forests prevail. 



Other reasons for this result also exist. Malarial fevers are much worse in the forest 

 than in the jungle, and are worse in the jungle than in the bush. The natives are said to 

 be immune to such fevers, but modern research throws considerable doubt on this. The 

 adults are immune, but how about the children? The researches of Sir Ronald Ross and 

 of the School of Tropical Medicine at Liverpool have shown that in coimtries badly infested 

 with malaria adults do not suffer much from the disease, but that nearlj^ half of the children 

 have it year after year during childhood, and a large number bear its marks through life 

 in the form of enlarged spleens and other injurious alterations of the organs. Every gener- 

 ation is apparently distinctly weakened by the diseases through which it passes in child- 

 hood. Similarly, in places such as Merida, where yellow fever is endemic, it is said that the 

 natives never suffer and that epidemics break out only when newcomers arrive from outside. 

 Many physicians now think, however, that large numbers of the children have the fever 

 in infancy. Those who die are supposed to have suffered from other infant complaints, 

 while those who recover are of course immune. In the case of j^ellow fever the after effects 

 are generally not serious, but in the case of malarial fevers, especially such forms as prevail 

 in the tropics, the debilitating results often last through life. Thus it may be that the 

 severe fevers of the forests, attacking the children and killing many of them, leave the 

 remainder permanently weakened and incapacitated for the work of forwarding civihzation 

 in their hard surroundings. 



In general, so far as the effects of climate upon human efficiency are concerned, there 

 seems to be a curious contradiction between equatorial and non-equatorial regions. In the 



