"Descent of Man," vol. i. p. 242; and "Animals and Plants under Cultivation," vol. ii. pp. 227-230. I have taken the examples siven from the same author.

of essential value to each species; we do not know why white terriers are more subject than darker-coloured ones to the attacks of the fatal distemper; why yellow-fleshed peaches in America suffer more from diseases than the white-fleshed varieties; why white chickens are most liable to the ga2)es; or why the caterpillars of silkworms, which produce white cocoons, are not attacked by fungus so much as those that produce yellow cocoons? Yet in all these cases, and many others, it has been shown that immunity from disease is correlated with some slight difference in colour or structure, but as to the cause of that immunity we are entirely ignorant. At last we reached the summit of the range, which is probably not less than three thousand feet above the sea, and entered on the district of Libertad. Hounded boggy hills covered with grass, sedgy plants and stunted trees, replaced the dry gravelly soil of the Juigalpa district. The low trees bore innumerable epiphytal plants, on their trunks and boughs. Many of these are species of Tillandsia, which sit perched up on the small branches like birds. They have sheathing leaves that hold at their base a supply of water that must be very useful to them in the diy season. Insects get drowned in this water, and the plants may derive some nourishment from their decomposing bodies, but I believe the principal object is to obtain a supply of moisture, as the roots of the plants do not hang down to the ground, like those of many other epiphytes in the tropics, nor are they provided with bulbs like the orchids. Some plants that hold liquids in cup-shaped leaves are simply insect traps, many of them growing in bogs, where the

iS4 THE NATURALIST IN NICARAGUA. [Ch. X,

supply of moisture is perennial and constant. Such is the Indian-cup (Sarracenia) that grows in the bogs of Canada, and the Californian pitcher-plant {Darlingtonia califorriica)^ which also grows in bogs, and is such an excellent fly-trap, that there is generally a layer of from two to five inches of decomposing insects lying at the bottom of the cup.* The different species of Brosera, or sun-dews, possess quite a different apparatus for catching insects, and they also live in bogs, which supports the inference that plants growing in such situations have some especial need to obtain nutriment, which they cannot draw from the decaying vegetation on which they live. Possibly they obtain the salts of potash in this way. I did not notice any provision in the leaves of the Bromeliaceous epiphytes of Chontales to ensure the capture of insects, but often saw their dead bodies in the water held at the base of the leaves, and any that came to drink would be very liable to slip into the water from off the nearly perpendicular side of the leaf and be drowned. It is not impossible that the small supply of mineral salts required for the organisation of these plants that do not draw any nutriment from the earth may be obtained from dead insects, but, as I have already stated, I believe that the principal object is to lay up a store of water to carry them safely through the diy season. Incidentally, the further advantage has been gained that insects fall into the receptacles of water and are drowned, affording in their decomposition nourishment to the plants.

Our road now lay over the damp grassy hills of the Libertad district. It edged away from the Amerrique * See "Nature," vol. iii. pp. 159 and 167.

rane-e on our riyht. To our left, about three miles distant, rose the dark sinuous line of the great forest of the Atlantic slope. Only a fringe of dark-foliaged trees in the foreground was visible, the higher ground behind was shrouded in a sombre pall of thick clouds that never lifted, but seemed to cover a gloomy and mysterious country beyond. Though I had dived into the recesses of these mountains again and again, and knew that they were covered with beautiful vegetation and full of animal life, yet the sight of that leadeu-coloured barrier of cloud resting on the forest tops, whilst the savannahs were bathed in sunshine, ever raised in my mind vague sensations of the unknown and the unfathomable. Our course was nearly parallel to this gloomy forest, but we gradually approached it. The line that separates it from the grassy savannahs is sinuous and irregular. In some places a dark promontory of trees juts out into the savannahs, in others a green grassy hill is seen almost surrounded by forest. When I first came to the country, I was much puzzled to understand why the forest should end just where it did. It is not because of any change in the nature of the soil or bed-rock. It cannot be for lack of moisture, for around Libertad it rains for at least six months out of the twelve. The surface of the ground is not level on the savannahs, but consists of hill and dale, just as in the forest. Altogether the conditions seemed to be exactly the same, and it appeared a difficult matter to account for the fact that the forest should end at an irregular but definite line, and that at that boundary grassy savannahs should commence. After seeing the changes that were wrought during the four and a half years that I was in

i86 THE NATURALIST IN NICARAGUA. [Ch. X.

the country, I have been led to the conclusion that the forest formerly extended much further towards the Pacific, and has been beaten back principally by the agency of man. The ancient Indians of Nicaragua were an agricultural race, their principal food then, as now, being maize; and in all the ancient graves, the stone for grinding corn is found placed there, as the one thing that was considered indispensable. They cut down patches of the forest and burnt it to plant their corn, as all along the edge of it they do still. The first time the forest is cut down, and the ground planted, the soil contains seeds of the forest trees, which, after the corn is gathered, spring up and regain possession of the ground, so that in twenty years, if such a spot is left alone, it will scarcely differ from the surrounding untouched forest. But it does not remain unmolested. After two or three years it is cut down again and a great change takes place. The soil does not now contain seeds of forest trees, and in their stead a great variety of weedy-looking shrubs, only found where the land has been cultivated, spring up. Grass, too, begins to get a hold on the ground; if it prevails, the Indian, or Mestizo, does not attempt to grow corn there again, as he knows the grass will spoil it, and he is too indolent to weed it out. Often, however, the brushwood has been cut down and burnt, and fresh crops of corn grown several times before the grass has gained such an advantage that the cultivator gives up the attempt to plant maize. There is then a struggle between the weedy shrubs and the grass. The leaf-cutting ants come to the aid of the latter. Grass they will not touch, excepting to clear it away from their paths.

The thick forest they do not like, possibly because beneath its shade the ground is kept too damp for their fungus beds. But along the edge of the forest, by the sides of roads through it, that let in the air and sunshine, and in clearings, they abound. They are especially fond of the leaves of young trees, many of which are destroyed by them. Should the brushwood ultimately prevail, and cover the ground, the Indian, or Mestizo, comes again after a few years, cuts it down, and replants it with maize. But as most of his old clearings get covered with grass, he is continually encroaching on the edge of the forest, beating it back gradually, but surely, towards the north-east. As this process has probably been going on for thousands of years, I believe that the edge of the forest is several miles nearer the Atlantic than it was originally.

In this way many acres in the neighbourhood of Pital were taken from the forest, and added to the grass-lands, whilst I was in the country. The brushwood-land does not yield such good crops as the virgin forest, but it is nearer to the huts of the cultivators, who live out on the savannahs, so that whenever the weedy shrubs gain possession of a spot sufficiently large for a clearing, and choke off the grass, these places are again cut down and burnt, and thus the forest is never allowed to establish outposts, or advanced stations, in the disputed ground. What would be the result if man were withdrawn from the scene, I do not know, but I believe that the forest would slowly, but surely, regain the ground that it has lost through long centuries. The thickets and dense brushwood that always spring up along the edge of the forest, and consist of many

l88 THE NATURALIST IN NICARAGUA. [Ch. X.

shrubs that the leaf-cutting ants do not touch, would gradually spread, and beat back the grass. In their shade and shelter, seeds from the forest would vegetate and grow, and thus, I think, very slowly, inch by inch, the forest would regain its long-lost territory, and gradually extend its limits towards the south-west, until it reached its old boundaries, where a change in the physical character of the land, or in the amount of moisture precipitated, would stay its further progress. It is far more likely, however, that man will drive back the forest to the very Atlantic than that he will quit the scene. After passing the Indian graves, about a league from Libertad, we turned off to the right, by a path that led directly to the Mico, without going through the town. After crossing several rounded grassy hills, we reached the river, and found it swollen with recent rains, but fordable. Sometimes travellers are detained several days, unable to cross, and I was always glad when, returning to the mines, I had put it behind me. Now and then a traveller is drowned when attempting to cross the swollen river, but these accidents are rare, as it is well known, by certain rocks being covered, when it is unfordable. If carried away, a traveller has little chance to save his life, as just below the crossing, the river is rapid and the banks precipitous. I heard of one man who had had a very narrow escape. He was trying to cross on muleback, but his beast lost its footing, rolled over, and was rapidly washed away. The poor man was carried into the roaring rapids, and would soon have been drowned, but a herdsman on the bank, who was looking for cattle, threw his lasso cleverly over the drowning traveller, and

dragged him on shore. Some of the " vacqneros," as the herdsmen are called, are wonderfully adroit in throwing the lasso; when riding at full speed, they throw it over the horns of the cattle, or the heads of the horses, and can hold the strongest if sideways on. But I have seen some old bulls that knew how to get loose; they would run straight away from the vacquero in places where he could not ride round them, and getting a straight pull on the lasso, would break it, or draw it out of his hands. There are no horses or mules, and very few cattle, however, that know how to do this, I was told by the herdsmen. After crossing the river, we soon reached Pital, where I had a cup of tea and got a fresh mule. We now turned nearly at right angles to our former course, and struck into the dark forest, the road through which I have already described. It was very wet and muddy. In some places, although it was only the commencement of the wet season, the mules sank above their knees. On this occasion, as on many others, I had often to notice how well the mule remembered places where in some former year it had avoided a particularly bad part by making a detour. I was riding a mule that had tender feet, having just recovered from the bite of a spider, that had occasioned the loss of one of its hoofs, and when it came near to a place where it could escape the deep mud by going over a stony part it would slacken its pace and look first at the mud, then at the stones, evidently balancing in its mind which was the lesser evil. Sometimes, too, when it came to a very bad place, which was better at the sides, I left it to itself, and it would be so undecided which side was the best, that makinsf

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