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ingly abundant, and very destructive to the fruit. In the next spring, numerous females that had hybernated commenced making their paper nests, and I anticipated a still greater plague of wasps in the autumn than we had had the year before; but some epidemic carried off nearly all the females before they finished building their nests, and in the autumn scarcely a wasp was to be seen; I saw also in the Natural History magazines notices of their scarcity in all parts of England. The great mortality amongst the insects of Chontales in 1872 has some bearing on the origin of species, for in times of such great epidemics we may suspect that the gradations that connect extreme forms of the same species may become extinct. Darwin has shown how very slight differences in the colour of the skin and hair are sometimes correlated with great immunity from certain diseases, and from the action of some vegetable poisons, and the attacks of certain parasites.” Any varieties of species of insects that could withstand better than others these great and probably periodical epidemics, would certainly obtain a great advantage over those not so protected; and thus the survival of one form, and the extinction of another, might be brought about. We see two species of the same genus, as in many insects, differing but little from each other, yet quite distinct, and we ask, why if these have descended from one parent form, do not the innumerable gradations that must have connected them exist also P. There is but one answer; we are ignorant what characters are
* “I)escent of Man,” vol. i. p. 242 ; and “Animals and Plants under Cultivation,” vol. ii. pp. 227-230. I have taken the examples given from the same author. .
Ch. X.] WATER RESERVOIRS OF PLANTS, 183
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 yellowfleshed peaches in America suffer more from diseases than the white-fleshed varieties; why white chickens are most liable to the gapes; or why the caterpillars of silkworms, which produce white cocoons, are not attacked by fungus, so much as those that produce yellow cocoons P. 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. Rounded 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 sheathy leaves that hold at their base a supply of water that must be very useful to them in the dry 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 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 californica), 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 Drosera, 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 dry season; and, 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,
Ch. X.] SHARP DELIMITATION OF FOREST. 185
range on our right. 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; and 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 leaden-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 caused by 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 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 with corn, 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 let 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 leafcutting ants come to the aid of the latter. Grass they will not touch, excepting to clear it away from their