CHAP. IV] MARSHY PLAINS 115 



among those that prey and those that are preyed on, 

 conceahng coloration has not been a survival factor ; 

 throughout the ages during which they have survived 

 they have gradually lost w^hatever of concealing colora- 

 tion they may once have had — if any — and have developed 

 a coloration which under present conditions has no con- 

 cealing and perhaps even has a revealing quality, and 

 which in aU probability never would have had a con- 

 cealing value in any " environmental complex " in which 

 the species as a whole lived during its ancestral develop- 

 ment. Indeed, it seems astonishing, when one observes 

 these big beasts — and big waders and other water-birds 

 — in their native surroundings, to find how utterly non- 

 harmful their often strikingly revealing coloration is. 

 Evidently the various other survival factors, such as 

 habit, and in many cases covert, etc., are of such over- 

 mastering importance that the coloration is generally 

 of no consequence whatever, one way or the other, and 

 is only very rarely a factor of any serious weight. 



The junction of the Sao Louren9o and the Paraguay 

 is a day's journey above Corumba. From Corumba 

 there is a regular service by shallow steamers to Cuyaba, 

 at the head of one fork, and to Sao Luis de Cdceres, at 

 the head of the other, l^he steamers are not powerful 

 and the voyage to each little city takes a week. There 

 are other forks that are navigable. Above Cuyaba and 

 Cdceres launches go up-stream for several days' journey, 

 except during the dry est parts of the season. North of 

 this marshy plain lies the highland, the Plan Alto, where 

 the nights are cool and the climate healthy. But I wish 

 emphatically to record my view that these marshy plains, 

 although hot, are also healthy ; and, moreover, the mos- 

 quitoes, in most places, are not in sufficient numbers to 

 be a serious pest, although, of course, there must be 



