The Naturalist in Nicaragua Part 18

The genus Calopteron belonging to this family is not phosph.o.r.escent. In some of the species, as in C. basalis (Klug), the wing-covers are widened out behind in a peculiar manner. This and other species of Calopteron are not only imitated in their colour and markings by other families of beetles, but also in this peculiar widening of the elytra. Besides this, the Calopteron when walking on a leaf raises and depresses its wing cases, and I observed exactly the same movement in a longicorn beetle (Evander n.o.bilis, Bates), which is evidently a mimetic form of this genus.

In addition to being mimicked by other families of beetles, Calopteron is closely resembled by a species of moth (Pionia lycoides, Walker). This moth varies itself in colour; in one of the varieties it has a central black band across the wings, when it resembles Calopteron vicinum (Deyrolle), in another this black band is wanting, when it resembles C. basalis. Professor Westwood has also pointed out to me that the resemblance to the beetle is still further increased in the moth by raised lines of scales running lengthwise down the thorax.

The phosph.o.r.escent species of Lampyridae, the fireflies, so numerous in tropical America, are equally distasteful, and are also much mimicked by other insects. I found different species of c.o.c.kroaches so much like them in shape and colour that they could not be distinguished without examination. These c.o.c.kroaches, instead of hiding in crevices and under logs like their brethren, rest during the day exposed on the surface of leaves, in the same manner as the fireflies they mimic.

Protective resemblances amongst insects are so numerous and widespread, and they have been so ably described by Bates and Wallace, that I shall only mention a few of the most noticeable examples that came under my attention, and which have not been described by other authors. Amongst these were the striking modifications of some beetles belonging to the Mordellidae. These, in their normal form, are curious wedge-shaped beetles, which are common on flowers, and leap like fleas. In some of the Nicaraguan species the body is lengthened, and the thorax and elytra coloured, so as to resemble wasps and flies. In the Mordellidae the head is small, and nearly concealed beneath the large thorax; and in the mimetic forms the latter is coloured so as to resemble the large head and eyes of the wasp or fly imitated. The species that resembles a wasp moves its antennae restlessly, like the latter insect.

The movements, as well as the shape and colour of the insect imitated, are mimicked. I one day observed what appeared to be a hornet, with brown semi-transparent wings and yellow antennae. It ran along the ground vibrating its wings and antennae exactly like a hornet, and I caught it in my net, believing it to be one. On examining it, however, I found it to belong to a widely different order. It was one of the Hemiptera, Spiniger luteicornis (Walk.), and had every part coloured like the hornet (Priocnemis) that it resembled. In its vibrating coloured wing-cases it departed greatly from the normal character of the Hemiptera, and a.s.sumed that of the hornets.

All the insects that have special means of protection, by which they are guarded from the attacks of insectivorous mammals and birds, have peculiar forms, or strongly contrasted, conspicuous colours, and often make odd movements that attract attention to them. There is no attempt at concealment, but, on the contrary, they appear to endeavour to make their presence known. The long narrow wings of the Heliconii b.u.t.terflies, banded with black, yellow, and red, distinguish them from all others, excepting the mimetic species. The banded bodies of many wasps, or the rich metallic colours of others, and their constant jerky motions, make them very conspicuous. Bees announce their presence by a noisy humming. The beetles of the genus Calopteron have their wing-cases curiously distended, and move them up and down, so as to attract attention; and other species of Lampyridae are phosph.o.r.escent, holding out danger signals that they are not eatable. The reason in all these cases appears to be the same as Mr. Wallace has shown to hold good with banded, hairy, and brightly coloured caterpillars.

These are distasteful to birds, and, in consequence of their conspicuous colours, are easily known and avoided. If they were like other caterpillars, they might be seized and injured before it was known they were not fit for food.* (* In a paper on "Mimicry, and other Protective Resemblances amongst Animals" first published in the "Westminster Review" July 1867, afterwards in "Natural Selection", Wallace has elaborately discussed this question. My observations are supplemental to his and to the original ones of Bates.)

(PLATE 23. HORNET AND MIMETIC BUG)

Amongst the mammals, I think the skunk is an example of the same kind. Its white tail, laid back on its black body, makes it very conspicuous in the dusk when it roams about, so that it is not likely to be pounced upon by any of the carnivora mistaking it for other night-roaming animals. In reptiles, the beautifully banded coral snake (Elaps), whose bite is deadly, is marked as conspicuously as any noxious caterpillar with bright bands of black, yellow, and red. I only met with one other example amongst the vertebrata, and it was also a reptile. In the woods around Santo Domingo there are many frogs. Some are green or brown, and imitate green or dead leaves, and live amongst foliage. Others are dull earth-coloured, and hide in holes and under logs. All these come out only at night to feed, and they are all preyed upon by snakes and birds. In contrast with these obscurely coloured species, another little frog hops about in the daytime dressed in a bright livery of red and blue. He cannot be mistaken for any other, and his flaming vest and blue stockings show that he does not court concealment. He is very abundant in the damp woods, and I was convinced he was uneatable so soon as I made his acquaintance and saw the happy sense of security with which he hopped about. I took a few specimens home with me, and tried my fowls and ducks with them, but none would touch them. At last, by throwing down pieces of meat, for which there was a great compet.i.tion amongst them, I managed to entice a young duck into s.n.a.t.c.hing up one of the little frogs. Instead of swallowing it, however, it instantly threw it out of its mouth, and went about jerking its head as if trying to throw off some unpleasant taste.* (* Probably the strongly contrasted colours of the spotted salamander of Southern Europe and the warning noise made by the rattlesnake may be useful in a similar manner, as has been suggested by Darwin.)

After travelling three leagues beyond Teustepe, we reached, near dusk, a small house by the roadside, at which had put up for the night a party of muleteers, with their mules and cargoes. Our beasts were too tired to go further, so we determined to take our chance of finding room for our hammocks. Soon after we alighted, as I sat on a stone near the door of the house, a gun went off close to us, and my horse sprang forward, nearly upon me. We soon found it was our own gun, which had been given to Rito to carry. He had strapped it behind his saddle, and one of the other mules had come up, rubbed against it, and let it off. The poor horse was only four feet from the muzzle, and the contents were lodged in its loin. A large wound was made from which the blood flowed in a great stream, until Velasquez got some burnt cloth and stanched it. Fortunately the charge in the gun was a very light one, and no vital part was touched. We arranged with the muleteers to take our cargo to Juigalpa for us, and determined to leave Rito behind to lead the horse gently to Pital. The horse, which was a very good one, ultimately recovered.

At this house the woman had eight children, the eldest, I think, not more than twelve years of age. The man who pa.s.sed as her husband was the father of the youngest only. Amongst the lower cla.s.ses of Nicaragua men and women often change their mates. In such cases the children remain with the mother, and take their surname from her. Baptism is considered an indispensable rite, but the marriage ceremony is often dispensed with; and I did not notice that those who lived together without it suffered in the estimation of their neighbours. The European ladies at Santo Domingo were sometimes visited by the unmarried matrons of the village, who were very indignant when they found that there were scruples about receiving them. They were so used to their own social observances, that they thought those of the Europeans unwarrantable prudery.

Before turning out the mules, Rito got some limes and squeezed the juice out upon their feet, just above the hoof. He did this to prevent them from being bitten by the tarantula spider, a species of Mygale that makes its nest in the ground, and is said to abound in this locality. Many of the mules are bitten in the feet on the savannahs by some venomous animal. The animal bitten immediately goes lame, and cannot be cured in less than six months, as the hoof comes off, and has to be renewed. The natives say that the Mygale is the aggressor; that it gets on the mule's foot to bite off the hairs to line its nest with, and that if not disturbed it does not injure the mule, but that if the latter tries to dislodge it, it bites immediately. I do not know whether this story be true or not, and I had no opportunity of examining a Mygale's nest to see if it was lined with hairs, but Professor Westwood informs me that all that he knows are lined with fine silk. Possibly the mules, when rambling about, step on the spider, and are then bitten by it.

Velasquez told me that when he was a boy he and other children used to amuse themselves by pulling the Mygale out of its hole, which is about a foot deep in the ground. To get it out they fastened a small ball of soft wax to a piece of string, and lowered it down the hole, jerking it up and down until the spider got exasperated so far as to bury its formidable jaws in the wax, when it could be drawn to the surface.

We had part of the kitchen to sleep in, and were so tired, and getting so accustomed to sleep anywhere, that we had a good night's rest, rose early next morning, and were soon on the road again, leaving Rito to bring on the lamed horse. We had a good view of the rock of San Lorenzo, a high cliff capping a hill, and resembling the rocks of Cuapo and Pena Blanca, but with less perpendicular sides. About this part, which lay high, as well as where we stayed the night before, there had been rains; but on the lowlands lying between the two places there had been none. Our road again lay over gra.s.sy plains and low, lightly-timbered hills, with very few houses--probably not more than one in a league. The country was now greener; they had had showers of rain, and fine gra.s.s had sprung up. Pa.s.sing as we did from a dried-up district into one covered with verdure, feelings were awakened akin to those with which in the temperate zone we welcome the spring after a long winter.

As we rode on, the gra.s.s increased; there were swampy places in the hollows, and now and then very muddy spots on the road. On every side the prospect was bounded by long ranges of hills--some of them precipitous, others covered to the summits with dark foliaged trees, looking nearly black in the distance. About noon we came in sight of the Amerrique range, which I recognised at once, and knew that we had reached the Juigalpa district, though still several leagues distant from the town. Travelling on without halting we arrived at the hacienda of San Diego at four o'clock. Velasquez expected to find in the owner an old acquaintance of his, and we had intended staying with him for the night, as our mules were tired out; but on riding up to the house we found it untenanted, the doors thrown down, and cattle stabling in it. We pushed on again. I thought I could make La Puerta, a hacienda three leagues nearer Libertad than Juigalpa, and as the road to it branched off from that to Juigalpa soon after pa.s.sing San Diego, and Velasquez had to go to the latter place to make arrangements for getting our luggage sent on, I parted with him, and pushed on alone. Soon after, I crossed rather a deep river, and in a short time my mule, which had shown symptoms of distress, became almost unable to proceed, so that it was only with the greatest difficulty I could get along at all. After leading--almost dragging--it slowly for about a mile I reached a small hut, where they told me that it was three leagues to La Puerta, and only one to Juigalpa. The road to Puerta was all up hill, and it was clearly impossible for me to reach it that night, so I turned off across the savannahs, in the direction of Juigalpa, wis.h.i.+ng that I had not separated from Velasquez. My poor beast was dragged along with much labour, and I was getting thoroughly knocked up myself. Several small temporary huts were pa.s.sed, in which lived families that had come down from the mountains, bringing with them their cows to feed on the plains during the wet season. I was tempted to put up at one of these, but all were full of people, and I persevered on until it got quite dark. Just then I arrived at a hacienda near the river, and engaged a young fellow to get his horse and ride with me to the town. When my mule had a companion it went better, and being very tired I got on its back again. It was extremely dark, and I should not have found the road without a guide. We pa.s.sed over the small plain, where the broken statues lie, but my guide, who had lived all his life within a mile of them, had never heard of them. My mule fell heavily with me in a rocky pa.s.s, but I escaped with a slight bruise. We had great trouble to get it on its legs again, and ultimately reached Juigalpa about nine o'clock.

Next morning I awoke with a dreadful headache and pain in my back, brought on either by the fatigue of the day before, or by having been tempted to eat some half-ripe guayavas when coming across the plains tired and hungry. I lay in the hammock until ten o'clock, and then feeling a little better, got on my mule and started. I was so ill as to be obliged to hold on to the pommel of my saddle and several times to get off and lie down. We had brought some "tiste"

with us made from chocolate and maize, and drinks of this relieved me. I at last reached Libertad at four o'clock, and went to bed immediately. Having fasted all day in place of taking medicine, I rose pretty well next morning, and we rode through the forest to the mines, reaching them at noon on the 29th July, after an absence of nineteen days.

CHAPTER 18.

Division of Nicaragua into three zones.

Journey from Juigalpa to lake of Nicaragua.

Voyage on lake.

Fresh-water sh.e.l.ls and insects.

Similarity of fresh-water productions all over the world.

Distribution of European land and fresh-water sh.e.l.ls.

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 princ.i.p.ally 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 princ.i.p.al products being india-rubber and mahogany. The central zone is composed of gra.s.sed 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 impa.s.sable, 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 pa.s.sage 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, gra.s.s, 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 gra.s.shoppers 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 sh.o.r.e, 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 pa.s.sed 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 t.i.tanic 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 sh.o.r.e were strewn sh.e.l.ls 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 sh.e.l.ls; 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 pa.s.sed 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.*

(* Darwin "Origin of Species" page 417.) 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, sh.e.l.ls, etc., and the dissimilarity of the surrounding terrestrial beings compared with those of Britain.* (*

Darwin "Origin of Species" page 414.) 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.

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 pa.s.s over arms of the sea, or fly from one country to another. Their sh.e.l.ls are the commonest of fossils; and in islands such as Madeira and St. Helena, where we have abundant remains of extinct land sh.e.l.ls, there are few, if any, of extinct animals of other cla.s.ses or of plants.

Taking the sh.e.l.ls 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 sh.e.l.ls 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."* (* 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 sh.e.l.ls 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-sh.e.l.ls 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 s.p.a.ce. 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 s.p.a.ce.

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 compet.i.tion 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 ill.u.s.trate more clearly the theory I have advanced, I will take a supposit.i.tious 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 compet.i.tion 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 cla.s.ses of terrestrial animals than in others. The land has often been submerged in geological history, and the cla.s.ses 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 mult.i.tudinous hosts of strong-flying insects; similarly, the mammals are far outnumbered by the birds of the air, that can pa.s.s from island to island, and from country to country, unstopped by mighty rivers or wide arms of the sea.

CHAPTER 19.

Iguanas and lizards.

Granada.

Politics.

Revolutions.

Cacao cultivation.

Masaya.

The lake of Masaya.