up-stream for several days’ journey, except during the dryest 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 mosquitoes, in most places, are not in sufficient numbers to be a serious pest, although of course there must be nets for protection against them at night.  The country is excellently suited for settlement, and offers a remarkable field for cattle-growing.  Moreover, it is a paradise for water-birds and for many other kinds of birds, and for many mammals.  It is literally an ideal place in which a field naturalist could spend six months or a year.  It is readily accessible, it offers an almost virgin field for work, and the life would be healthy as well as delightfully attractive.  The man should have a steam-launch.  In it he could with comfort cover all parts of the country from south of Corumbra to north of Cuyaba and Caceres.  There would have to be a good deal of collecting (although nothing in the nature of butchery should be tolerated), for the region has only been superficially worked, especially as regards mammals.  But if the man were only a collector he would leave undone the part of the work best worth doing.  The region offers extraordinary opportunities for the study of the life-histories of birds which, because of their size, their beauty, or their habits, are of exceptional interest.  All kinds of problems would be worked out.  For example, on the morning of the 3rd, as we were ascending the Paraguay, we again and again saw in the trees on the bank big nests of sticks, into and out of which parakeets were flying by the dozen.  Some of them had straws or twigs in their bills.  In some of the big globular nests we could make out several holes of exit or entrance.  Apparently these parakeets were building or remodelling communal nests; but whether they had themselves built these nests, or had taken old nests and added to or modified them, we could not tell.  There was so much of interest all along the banks that we were continually longing to stop and spend days where we were.  Mixed flocks of scores of cormorants and darters covered certain trees, both at sunset and after sunrise.  Although there was no deep forest, merely belts or fringes of trees along the river, or in patches back of it, we frequently saw monkeys in this riverine tree-fringe—­active common monkeys and black howlers of more leisurely gait.  We saw caymans and capybaras sitting socially near one another on the sandbanks.  At night we heard the calling of large flights of tree-ducks.  These were now the most common of all the ducks, although there were many muscovy ducks also.  The evenings were pleasant and not hot, as we sat on the forward deck; there was a waxing moon.  The screamers were among the most noticeable birds.  They were noisy; they perched on the very tops of the trees, not down among the branches; and they were not shy.  They should be carefully protected by law, for they readily become tame, and then come familiarly round the houses.  From the steamer we now and then saw beautiful orchids in the trees on the river bank.