TROPICAL CLEVIATES AND BIOLOGY — CARTER 433 



Between the two extremes of climate that I have discussed there is, 

 of course, a very wide range of intermediate tropical environments. 

 These extend from woodland of many types to grassland and savanna, 

 and to arid scrub where desert conditions are approached. In their 

 general distribution this series of environments follows the reduction 

 of rainfall as one goes north or south from the Equator, but every- 

 where conditions are greatly modified by the local geography. Near 

 the sea, and especially where trade winds blow onto the land, rain 

 is more plentiful than farther inland; the monsoon modifies climate 

 in some countries; mountains may precipitate rain on their windward 

 sides and produce deserts in their lee; and many still more local fea- 

 tures of the geography, such as the nature of the subsoil and the 

 amount of percolation it allows, or the efficiency of the surface drain- 

 age, will modify the environment in smaller areas. 



As a first example of work on tropical biology that has given results 

 not to be expected from our knowledge of the biology of temperate 

 regions, I will take work on the conditions of life in shallow and 

 stagnant fresh waters. Such environments are very widely distributed 

 in the Tropics. Mangrove swamps are found near the banks of many 

 of the rivers, and papyrus swamps are widespread in Africa not only 

 bordering the rivers and lakes but also filling shallow valleys far from 

 the lakes. (In parts of Uganda 30 percent of the land is said to be 

 under papyrus.) In rainforests large areas may be permanently 

 flooded along the banks of the rivers, stretching many miles into the 

 forest, and, besides all these, swamps of many kmds are found in open 

 country. 



Some of the features of these swamps are common to most of them. 

 Almost always the water lies under thick growths of aerial vegeta- 

 tion — trees in the mangrove swamps and forest, papyrus which may 

 grow to 12 to 15 feet high, and in the swamps of open country, grasses 

 and other plants almost equally high. The water is often highly 

 colored — it may have the color of weak tea — and is almost or quite 

 stagnant even in the mangrove and papyrus swamps on the borders 

 of rivers and lakes. In temperate countries undisturbed by man 

 swamps may be equally widespread, but the conditions of life in their 

 water are, as we shall see, very different from those in tropical swamps. 



(My own interest in these environments has centered in the fact 

 that they are of great interest for the study of evolution. It was al- 

 most certainly from swamps of this kind that vertebrates and probably 

 many other terrestrial animals emerged from the water. But I shall 

 not have space to discuss these matters in this paper.) 



I take as a first example of tropical swamps some in the Paraguayan 

 Chaco in South America, in which Professor Beadle and I worked [2] . 



