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Ch. XVIII.) EMBARK TO CROSS THE LAKE.

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small flocks amongst the bushes; and the “sanaté ? (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 do not encroach on each other's side.

We stopped at a little hacienda perched on the top of a small hill. It was called “El Candelera,” and was a small cattle station, surrounded by plains. We now 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 Santo Claro, a cattle hacienda, at dusk. Here we found Señor 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 re

Ch. XVIII.]

FRESH-WATER ANIMALS.

331

sembled 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 Ampullaria.

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 waterbeetles of Europe. Gyrimide swam round and round in mazy circles ; Dytiscidæ 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 Hydrophilile crawled about, just as in ponds in England. Not only were those old-world familiars there, but they were represented by species belonging to the typical generaGyrinus, 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 world is not confined to animal life, but extends to plants also. Alph. 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; and 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.* 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, &c., and the dissimilarity of the surrounding terrestrial beings compared with those of Britain. † 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 Dytiscidæ. 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 Dytiscidæ is of special interest when we 'recollect that they are nothing but Carabide fitted for swimming, and yet that the Carabidæ are one of the groups in which the tropical members differ widely from the temperate ones.

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 dif

Darwin, “ Origin of Species,” p. 417. Ibid. p. 414.

Ch. XVIII.] LAND AND FRESH-WATER MOLLUSKS.

333

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ference in the distribution of the land and freshwater 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 Britain, 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 Lymnæacea there are not six species, it may be 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 Thibet. 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 Lymnæa, 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." * * Lovell Reeve, “ British Land and Fresh-Water Mollusks,” p. 255.

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