Division of Nicaragua into three zones. Journey from Juigalpa to lake of Nicaragua. Voyage on lake. Fresh-water shells and insects. Similarity of fresh-water productions all over the world. Distribution of European land and fresh-water shells. Discussion of the reasons why fresh-water productions have varied less than those of the land and of the sea.
I SHALL ask my readers to accompany me on one more journey. I have described the great Atlantic forest that clothes the whole of the eastern side of Nicaragua. I have gone through the central provinces, Chontales, Matagalpa, and Segovia; from the San Juan river, the south-eastern boundary of Nicaragua, away to the confines of Honduras on the north-west. I now propose to leave the central provinces, amongst which we have so long lingered, and to describe one of my journeys to those lying between the great lakes and the Pacific.
Whilst the country to the north-east of the lakes is mostly composed of rocks, of great age, geologically, such as schists, quartzites, and old dolerytic rocks, with newer but still ancient trachytes, that to the south-west of them is formed principally of recent volcanic tufas and lavas, the irruption of which has not yet ceased. Most of the land, resulting from the decomposition of the tufas, is of extreme fertility; and, therefore, we find on the Pacific side of Nicaragua, indigo, coffee, sugar, cacao, and tobacco growing with the greatest luxuriance.
Nicaragua is thus divided into three longitudinal zones. The most easterly is covered by a great unbroken forest; the principal products being india-rubber and mahogany. The central zone is composed of grassed savannahs, on which are bred cattle, mules, and horses. It is essentially a pasturage country, though much maize and a little sugar and indigo are grown in some parts. The western zone skirts the Pacific, and is a country of fertile soil, where all the cultivated plants and fruits of the tropics thrive abundantly; the rich, fat land might, indeed, with a little labour, be turned into a Garden of Eden.
In the autumn of 1871, it became necessary for me to proceed to Granada to empower a lawyer there to act for us in a lawsuit in which we were engaged. Taking Velasquez and a servant with me, I rode over to Juigalpa on the 1st of November. We had intended to go by land to Granada, but we learnt that, through continued wet weather, much of the low land of the delta of the Malacatoya was impassable, so we determined to make for the lake, and try to get a boat to take us to Los Cocos, from which place there was a good road to Granada. We found at Juigalpa a Libertad storekeeper, named Senor Trinidad Ocon. He had already engaged a boat, and courteously offered, if we could not find one when we got to the lake, to give us a passage in his.
We started from Juigalpa the next morning; and for the first few miles our road lay down by the river, a deep branch of which we crossed. The alluvial plains bordering the river were covered with fine, though short, grass, amongst which were some beautiful flowers. The orange and black “sisitote” (Icterus pectoralis, Wagl. ) flew in small flocks amongst the bushes; and the “sanate” (Quiscalus) was busy amongst the cattle. Their usual plan of operations is for a pair of them to accompany one of the cattle, one on each side, watching for grasshoppers and other insects that are frightened up by the browsing animal. They keep near the head, and fly after the insects that break cover, but neither encroaches on the hunting ground of the other.
We stopped at a little hacienda perched at the top of a small hill. It was called “El Candelera,” and was a small cattle station, surrounded by plains. We then crossed the valley, and made for a range of hills between us and the lake. The ascent was steep and rocky; and it took us two hours to get to the top. We then saw the great lake, like a sea, lying spread out before us, but still at a considerable distance. The descent was very steep, and we had to make long detours to avoid precipitous ravines. At last we reached level ground; but it was even worse than the mountain roads to travel, being in many parts wet and swampy. After missing our way, and having to retrace our steps for more than a mile, we reached Santa Claro, a cattle hacienda, at dusk. Here we found Senor Ocon’s boat, but there was no other. The boatmen said we must embark at once. We made an arrangement with a man who had accompanied Ocon to take our mules to San Ubaldo, as we proposed to return that way. The boat was small, and there were seven of us; so that with our saddles and luggage we were much cramped for room.
They poled the boat for two miles down a small river that emptied into the lake, but just before we reached it, the boatmen stopped and said it was too rough to proceed that night, and notwithstanding our remonstrances they tied the boat to some bushes. Our cramped position was very irksome; the river was bordered by swamps, so that we could not land, and thousands of mosquitoes came about and rendered sleep impossible. About midnight, the moon rose, and two hours later we prevailed on the boatmen to set sail, but, notwithstanding their excuse about it being too rough, there was so little wind that we made slow progress. At eight we went on shore, where there was a hut built close by the lake below Masaya. The lake was flooded, and the water had been over the floor of the hut during the night. All around were swamps, and the mosquitoes were intolerable. We could buy no food at the miserable shanty, and soon set sail again. A little more wind afterwards springing up, we reached Los Cocos at eleven o’clock. There is a small village at this place, where we got breakfast cooked, and did justice to it. We hired horses to take us to Granada; but as the road for a league further on was overflown by the lake, we went on in the boat, and a boy took the horses round to meet us, swimming them across the worst places.
Glad we were to get on horseback again, and to canter along a hard sandy road, instead of sitting cramped up in a little boat, with the sun’s rays pouring down on us. The path led amongst the bushes, and was sometimes overflowed, but the soil was sandy, and there was no mud. All the beach was submerged, or we should have ridden along it. The last time I had passed by this part of the lake was in July 1868. Then the waters of the lake were low, and we rode along the sandy beach, black in some parts with titanic iron sand. The beach resembled that of a sea-coast, with the waves rolling in upon it, and to the south-east the water extended to the horizon. Along the shore were strewn shells thrown up by the surf; and on examining them, I found them all to belong to well-known old-world genera — Unio, Planorbis, Ancylus, and Ampullari.
On this journey, all the beach was, as I have said, covered with water, and I saw no shells; but in the pools on the road were water-beetles swimming about, and these showed a surprising resemblance to the water-beetles of Europe. Gyrinidae swam round and round in mazy circles; Dytiscidae came up to the surface for a moment, and dived down again to the depths below with a globule of air glistening like a diamond. Amongst the vegetation at the bottom and sides of the pools Hydrophilidae crawled about, just as in ponds in England. Not only were those familiars there, but they were represented by species belonging to the typical genera — Gyrinus, Colymbetes, and Hydrophilus. Over these pools flew dragon-flies, whose larval stages are passed in the water, closely resembling others all over the world. All the land fauna was strikingly different from that of other regions; but the water fauna was as strikingly similar.
The sameness of fresh-water productions all over the globe is not confined to animal life, but extends to plants also. Alphonse de Candolle has remarked that in large groups of plants which have many terrestrial and only a few aquatic species the latter have a far wider distribution than the former. It is well known to botanists that many fresh-water and marsh plants have an immense range over continents, extending even to the most remote islands.1 The close affinities of fresh-water animals and plants have been noticed by many naturalists. Darwin saw with surprise, in Brazil, the similarity of the fresh-water insects, shells, etc., and the dissimilarity of the surrounding terrestrial beings compared with those of Britain.2 Dr. D. Sharp informs me that water-beetles undoubtedly present the same types all over the world. He believes there is no family of Coleoptera in which tropical or extra-tropical species so closely resemble one another as in the Dytiscidae. Cybister is found in Europe, Asia, Africa, Australia, New Zealand, Brazil, and North America; and the species have a very wide range. Dr. Sharp remarks that this wide distribution and great similarity of the Dytiscidae is of special interest when we recollect that they are nothing but Carabidae fitted for swimming, and yet that the Carabidae are one of the groups in which the tropical members differ widely from the temperate ones.
1 Darwin “Origin of Species” page 417.
2 Darwin “Origin of Species” page 414.
For following up this branch of inquiry the study of the distribution of the mollusca offers special advantages. There are numerous marine, fresh-water, and terrestrial species and genera. They are slow moving; they have not the means of transporting themselves great distances, like insects, for example, that may easily and often pass over arms of the sea, or fly from one country to another. Their shells are the commonest of fossils; and in islands such as Madeira and St. Helena, where we have abundant remains of extinct land shells, there are few, if any, of extinct animals of other classes or of plants.
Taking the shells of Europe, we find a remarkable difference in the distribution of the land and fresh-water species. According to Mr. Lovell Reeve, who has specially studied this question, out of many hundreds of land mollusks inhabiting the Caucasian province at its centre in Hungary and Austria, only ninety extend to the British Isles, and of these thirty-five do not reach Scotland. Upwards of two hundred species of Clausilia are to be found in the centre of the province, and of these only four reach England, and only one Scotland. Out of five hundred and sixty species of Helix inhabiting the Caucasian province, there are but twenty-four in Britain.
Whilst the distribution of the terrestrial mollusks of Europe is thus restricted in range, though the species are numerous, the fresh-water shells are few in species, but of wide distribution. Quoting again from Mr. Reeve:— Of the Lymnaeacea “there are not six species, it maybe safely stated, in all Europe, more than there are in Britain. They have no particular centre of creation. There is no evidence to show whether the alleged progenitors of our British species were created in Siberia, Hungary, or Tibet. There is scarcely any variation either in the form or number of the species in those remote localities. Of Planorbis scarcely more than fifteen species inhabit the whole Caucasian province, and we have eleven of them in Britain.” “Of Physa and Lymnaea, it is extremely doubtful whether there are any species throughout the province more than we have in Britain. Neither of Ancylus, which lives attached, limpet-like, to sticks and stones, and has very limited facilities of migration, are there any species throughout the province more than we have in Britain.”1
1 Lovell Reeve “British Land and Fresh–Water Mollusks” page 225.)
The wide distribution of species inhabiting fresh water compared with those living on land has not, as we have seen, escaped the comprehensive mind of Darwin, and in explanation of the fact, he has shown how fresh-water shells may be carried from pool to pool, or from one river or lake to others many miles distant, sticking to the feet of water-fowl, or to the elytra of water-beetles. Whilst the distribution of water-mollusks may be thus accounted for, the greater variety and more restricted range of the land species is not explained. They have at least equal means of dispersion, compared with the sluggish, mud-loving water-shells of our ponds and ditches. Why should the one have varied so much and the other so little? We might at first sight have expected the very reverse, on the theory of natural selection. In large lakes and in river systems isolated from others, we might look for the conditions most favourable for the variation of species, and for the preservation of the improved varieties.
It is evident that there must have been less variation, or that the varieties that arose have not been preserved. I think it probable that the variation of fresh-water species of animals and plants has been constantly checked by the want of continuity of lakes and rivers in time and space. In the great oscillations of the surface of the earth, of which geologists find so many proofs, every fresh-water area has again and again been destroyed. It is not so with the ocean — it is continuous — and as one part was elevated and laid dry, the species could retreat to another. On the great continents the land has probably never been totally submerged at any one time; it also is continuous over great areas, and as one part became uninhabitable, the land species could in most cases retreat to another. But for the inhabitants of lakes and rivers there was no retreat, and whenever the sea overflowed the land, vast numbers of fresh-water species must have been destroyed. A fresh-water fauna gave place to a marine one, and the former was annihilated so far as that area was concerned. When the land again rose from below the sea, the marine fauna was not destroyed — it simply retired farther back.
There is every reason to believe that the production of species is a slow process, and if fresh-water areas have not continued as a rule through long geological periods, we can see how variation has been constantly checked by the destruction, first in one part, then in another, of all the fresh-water species; and on these places being again occupied by fresh water they would be colonised by forms from other parts of the world. Thus species of restricted range were always exposed to destruction because their habitat was temporary and their retreat impossible, and only families of wide distribution could be preserved. Hence I believe it is that the types of fresh-water productions are few and world-wide, whilst the sea has mollusks innumerable, and the land great variety and wealth of species. This variety is in the ratio of the continuity of their habitats in time and space.
It follows also, from the same reasoning, that old and widespread types are more likely to be preserved in fresh-water areas than on land or in the sea, for the destruction of wide-ranging species is effected more by the competition of improved varieties than by physical causes; so that when variation is most checked old forms will longest survive. Therefore I think it is that amongst fishes we find some old geological types still preserved in a few of the large rivers of the world.
To illustrate more clearly the theory I have advanced, I will take a supposititious case. In the southern states of America there is reason to suppose that since the glacial period there has been a great variation in the species of the fresh-water mollusk genus Melania, and in different rivers there are distinct groups of species. Now let us suppose that the glacial period were to return, and that the icy covering, gradually thickening in the north, should push down southward as it did once before. The great lakes of North America would be again filled with ice, and their inhabitants destroyed. As the ice advanced southward, the inhabitants of one river-system after another would be annihilated, and many groups of Melania entirely destroyed. On the retreat of the ice again the rivers and lakes would reappear, but the varieties of animals that had been developed in them would not, and their places would be taken by aquatic forms from other areas, so that the number of species would be thereby greatly reduced, and wide-spreading forms would be freed from the competition of many improved varieties.
Viewed in this light, the similarity of fresh-water productions all over the world, instead of being a difficulty in the way of the acceptance of the theory of natural selection, becomes a strong argument in favour of its truth; for we perceive that the number of marine, terrestrial, and freshwater animals is in proportion to the more or less continuous development that was possible under the different conditions under which they lived.
The same line of argument might be used to explain the much greater variety in some classes of terrestrial animals than in others. The land has often been submerged in geological history, and the classes that were best fitted to escape the impending catastrophes would be most likely to preserve the varieties that had been developed. The atmosphere has always been continuous, and the animals that could use it as a highway had great advantages over those that could not, and so we find the slow-moving terrestrial mollusks few in number compared with the multitudinous hosts of strong-flying insects; similarly, the mammals are far outnumbered by the birds of the air, that can pass from island to island, and from country to country, unstopped by mighty rivers or wide arms of the sea.
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