Part 26 (1/2)
In late autumn or in winter the salmon sp.a.w.n in the rivers. The female makes a shallow trough in the gravel by moving her tail from side to side, and therein lays many eggs. The male, who is in attendance, fertilises these with the milt, and then the female covers them deeply with gravel. The process is repeated over and over again for a week or more till all the eggs are shed. For three to four months the eggs develop, and eventually there emerge the larvae or _alevins_, which lurk among the pebbles. They cannot swim much, for they are enc.u.mbered by a big legacy of yolk. In a few weeks, perhaps eight, the protruding bag of yolk has disappeared and the _fry_, about an inch long, begin to move about more actively and to fend for themselves. By the end of the year they have grown to be rather trout-like _parr_, about four inches long.
In two years these are double that length. Usually in the second year, but it may be earlier or later, the parr become silvery _smolts_, which go out to sea, usually about the month of May. They feed on young herring and the like and grow large and strong. When they are about three and a half years old they come up the rivers as _grilse_ and may sp.a.w.n. Or they may pa.s.s through the whole grilse stage in the sea and come up the rivers with all the characters of the full-grown fish. In many cases the salmon sp.a.w.n only once, and some (they are called _kelts_ after sp.a.w.ning) are so much exhausted by starting a new generation that they die or fall a victim to otters and other enemies. In the case of the salmon of the North Pacific (in the genus _Oncorhynchus_, not _Salmo_) all the individuals die after sp.a.w.ning, none being able to return to the sea. It must be remembered that full-grown salmon do not as a rule feed in fresh water, though they may be unable to resist snapping at the angler's strange creations. A very interesting fact is that the salmon keeps as it were a diary of its movements, which vary a good deal in different rivers. This diary is written in the scales, and a careful reading of the concentric lines on the scales shows the age of the fish, and when it went out to sea, and whether it has sp.a.w.ned or not, and more besides.
Interpretation of the Salmon's Story
When an animal frequents two different haunts, in one of which it breeds, it is very often safe to say that the breeding-place represents the original home. The flounder is quite comfortable far up the rivers, but it has to go to the sh.o.r.e-waters to sp.a.w.n, and there is no doubt that the flounder is a marine fish which has recently learned to colonise the fresh waters. Its relatives, like plaice and sole, are strictly marine. But it is impossible to make a dogma of the rule that the breeding-place corresponds to the original home. Thus some kinds of ba.s.s, which belong to the marine family of sea-perches, live in the sea or in estuaries, while two have become permanent residents in fresh water. Or, again, the members of the herring family are very distinctively marine, but the shad, which belong to this family, sp.a.w.n in rivers and may spend their lives there.
So there are two different ways of interpreting the life-history of the salmon. Some authorities regard the salmon as a marine fish which is establis.h.i.+ng itself in fresh water. But others read the story the other way and regard the salmon as a member of a freshwater race, that has taken to the sea for feeding purposes. In regard to trout, we know that the ranks of those in rivers and lakes are continually being reinforced by migrants from the sea, and that some trout go down to the sea while others remain in the freshwater. We know also in regard to a related fish, the char, that while the great majority of kinds are now permanent residents in cold and deep, isolated northern lakes, there are Arctic forms which live in the sea but enter the rivers to sp.a.w.n. These facts favour the view that the salmon was originally a marine fish. But there are arguments on both sides, and, for our present purpose, the important fact is that the salmon is conquering _two_ haunts. Its evolution is going on.
The Romance of the Eel
Early in summer, at dates varying with the distance of the rivers from the open Atlantic, crowds of young eels or elvers come up-stream.
Sometimes the procession or eel-fare includes thousands of individuals, each about the length of our first finger, and as thick as a stout knitting needle. They obey an inborn impulse to swim against the stream, seeking automatically to have both sides of their body equally stimulated by the current. So they go straight ahead. The obligation works only during the day, for when the sun goes down behind the hills the elvers snuggle under stones or beneath the bank and rest till dawn.
In the course of time they reach the quiet upper reaches of the river or go up rivulets and drainpipes to the isolated ponds. Their impulse to go on must be very imperious, for they may wriggle up the wet moss by the side of a waterfall or even make a short excursion in a damp meadow.
In the quiet-flowing stretches of the river or in the ponds they feed and grow for years and years. They account for a good many young fishes.
Eventually, after five or six years in the case of the males, six to eight years in the case of the females, the well-grown fishes, perhaps a foot and a half to two feet long, are seized by a novel restlessness.
They are beginning to be mature. They put on a silvery jacket and become large of eye, and they return to the sea. In getting away from the pond it may be necessary to wriggle through the damp meadow-gra.s.s before reaching the river. They travel by night and rather excitedly. The Arctic Ocean is too cold for them and the North Sea too shallow. They must go far out to sea, to where the old margin of the once larger continent of Europe slopes down to the great abysses, from the Hebrides southwards. Eels seem to sp.a.w.n in the deep dark water; but the just liberated eggs have not yet been found. The young fry rises to near the surface and becomes a knife-blade-like larva, transparent all but its eye. It lives for many months in this state, growing to be about three inches long, rising and sinking in the water, and swimming gently.
These open-sea young eels are known as Leptocephali, a name given to them before their real nature was proved. They gradually become shorter, and the shape changes from knife-blade-like to cylindrical. During this change they fast, and the weight of their delicate body decreases. They turn into gla.s.s-eels, about 2-1/2 inches long, like a knitting-needle in girth. They begin to move towards the distant sh.o.r.es and rivers, and they may be a year and a half old before they reach their destination and go up-stream as elvers. Those that ascend the rivers of the Eastern Baltic must have journeyed three thousand miles. It is certain that no eel ever matures or sp.a.w.ns in fresh water. It is practically certain that all the young eels ascending the rivers of North Europe have come in from the Atlantic, some of them perhaps from the Azores or further out still. It is interesting to inquire how the young eels circ.u.mvent the Falls of the Rhine and get into Lake Constance, or how their kindred on the other side of the Atlantic overcome the obstacle of Niagara; but it is more important to lay emphasis on the variety of habitats which this fish is trying--the deep waters, the open sea, the sh.o.r.e, the river, the pond, and even, it may be, a little taste of solid earth. It seems highly probable that the common eel is a deep-water marine fish which has learned to colonise the freshwaters. It has been adventurous and it has succeeded. The only shadow on the story of achievement is that there seems to be no return from the sp.a.w.ning. There is little doubt that death is the nemesis of their reproduction. In any case, no adult eel ever comes back from the deep sea. We are minded of Goethe's hard saying: ”Death is Nature's expert advice to get plenty of life.”
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Forming New Habits
There is a well-known mudfish of Australia, Neoceratodus by name, which has turned its swim-bladder into a lung and comes to the surface to spout. It expels vitiated air with considerable force and takes fresh gulps. At the same time, like an ordinary fish, it has gills which allow the usual interchange of gases between the blood and the water. Now this Australian mudfish or double-breather (Dipnoan), which may be a long way over a yard in length, is a direct and little-changed descendant of an ancient extinct fish, Ceratodus, which lived in Mesozoic times, as far back as the Jura.s.sic, which probably means over five millions of years ago. The Queensland mudfish is an antiquity, and there has not been much change in its lineage for millions of years. We might take it as an ill.u.s.tration of the inertia of evolution. And yet, though its structure has changed but little, the fish probably ill.u.s.trates evolution in process, for it is a fish that is learning to breathe dry air. It cannot leave the water; but it can live comfortably in pools which are foul with decomposing animal and vegetable matter. In partially dried-up and foul waterholes, full of dead fishes of various kinds, Neoceratodus has been found vigorous and lively. Unless we take the view, which is _possible_, that the swim-bladder of fishes was originally a lung, the mud-fishes are learning to breathe dry air. They ill.u.s.trate evolution agoing.
[Ill.u.s.tration: DIAGRAM OF THE LIFE HISTORY OF THE COMMON EEL (_Anguilla Vulgalis_)
1. The transparent open-sea knife-blade-like larva called a Leptocephalus.
2 and 3. The gradual change of shape from knife-blade-like to cylindrical. The body becomes shorter and loses weight.
4. The young elver, at least a year old, which makes its way from the open sea to the estuaries and rivers. It is 2/3 inches long and almost cylindrical.
5. The fully-formed eel.]
[Ill.u.s.tration: _Photo: Gambier Bolton._
Ca.s.sOWARY
Its bare head is capped with a helmet. Unlike the plumage of most birds its feathers are loose and hair-like, whilst its wings are merely represented by a few black quills. It is flightless and entirely dependent on its short powerful legs to carry it out of danger.]
[Ill.u.s.tration: _Photo: Gambier Bolton._