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.*
(* 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, shells, 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 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."* (* 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.