Popular Science Monthly/Volume 52/April 1898/Migration
By W. K. BROOKS, LL. D.,
PROFESSOR OF ZOÖLOGY IN THE JOHNS HOPKINS UNIVERSITY.
IT is easy to understand how natural selection may modify organisms for the good of the species, even at the expense of the individuals which, in each generation, make up the species; but it is difficult to understand how this can be brought about by nurture, for, so far as the direct action of the conditions of life is concerned, the species is identical with the sum of the individuals which now exist. No illustration of the law that the adaptations of living Nature are for the good of the species, and that when this comes into conflict with the welfare of the individuals, these are sacrificed, is more simple or more easy to understand than that afforded by some of the phenomena of migration.
The young salmon which is born in a mountain stream is soon impelled, by something in its nature, to journey downward, even for many hundred miles, until it reaches the unknown ocean, where it would discover, if it had faculties for anything so subjective as discovery, that, while it was born in a little brook, it was made for life in the great ocean. It has brought from its mountain home a natural aptitude for eluding all the strange enemies and for avoiding all the novel dangers which it meets in this new world, and it leads an active, predatory life, fiercely pursuing and destroying its natural but hitherto unknown prey; for growing rapidly, and quickly acquiring all the characteristics of the adult salmon, and storing up the intense nervous energy and the muscular strength which will be needed for forcing its way up the rapids in the mountain torrents, for leaping waterfalls, and fighting for its passage, where it long ago darted down with the current. As sexual maturity approaches, some stimulus, which has its origin in the developing reproductive organs, impels it to leave the ocean and, entering the mouth of a river, to journey upward, often a thousand miles or more, to its sources in the mountains.
At this time the king of fishes, as it is well called, is in physical perfection, with few rivals in beauty or strength or fierce energy, or indomitable courage and perseverance; but its strength is soon fully taxed in surmounting the obstacles and in fighting the rivals which oppose its progress, until at last, worn and thin, torn and mangled by battle, and battered by rocks and whirlpools, with its skin in rags, its fins crippled and bleeding, and its whole body from nose to tail bruised and emaciated, nothing of its kingly nature remains except the indomitable impulse, which no hardship can quench, still urging it upward, until, if any life is left, it at last reaches the breeding ground.
One of the most magnificent species of this kingly genus was so abundant in the Columbia River before canning houses had reduced its numbers, that the town reaches were packed with salmon, while the surface was covered with the drifting bodies of those that had perished in fierce struggles with the crowd; yet there is good authority for the assertion that not a single one ever returns alive from the breeding grounds in the head waters of the St. Cloud. The whole race is wiped out, utterly exterminated, as soon as it arrives at maturity and physical perfection, in order that the perpetuation of the species may be assured. The whole object and end of the beautifully co-ordinated body, which is provided for by such admirable and wonderful adaptations, which is built up so slowly and at so much cost, is rapid and total destruction.
The marvelous instinct which leads the young fish to the ocean, the organization and the habits which fit it for its marine life—all, in a word, which makes of the salmon our ideal of a lordly fish—is worth nothing as compared with the welfare of generations yet unborn.
Scientific men who are not zoölogists are fond of telling us that science has nothing to do with the Why? and is only concerned with the How? and while this may perhaps be true in the ultimate or philosophical sense of the words, it is often easy in zoölogy to discern why an action is performed, while we are very ignorant of the structural conditions under which it takes place.
As all the individual California salmon seem to act alike, and as the young salmon has no parental instruction, it seems probable that everything it does is the result of its structure or of such nurture as this structure provides for; and yet we may safely say that no one now living is at all likely to discover or to predict its migration from the study of its body, although the reason why the migration takes place is obvious.
Whole books, and not a few of them, have been devoted to learned speculations on the nature of the impulse which leads to the migration of birds, and while the subject is most fascinating, the value of the results has not in all cases paid for the labor. Newton (Encyclopæedia Britannica, article Birds) says: "We have here more than enough to excite our wonder, and instead are brought face to face with perhaps the greatest mystery which the whole animal kingdom presents—a mystery which attracted the attention of the earliest writers, and can in its chief point be no more explained by the modern man of science than by the simple-minded savage or the poet or prophet of antiquity. Some facts are almost universally known, and have been the theme of comment in all ages and in all lands. The hawk that stretches her wings toward the south is as familiar to the latest Nile-boat traveler or dweller on the Bosporus as of old to the author of the book of Job.
"The autumnal thronging by myriads of waterfowl of the rivers of Asia is witnessed by the modern sportsman as it was of old by Homer. . . . But there is no need to multiply instances. The flow and ebb of the mighty feathered wave has been sung by poets and reasoned by philosophers, has given rise to proverbs and entered into popular superstitions, and yet we must say of it still that our ignorance is immense."
While this author does not exaggerate either the interest or our ignorance of the life of birds, which goes on in regions that are almost inaccessible and unknown, there is no reason to suppose their migrations are any more mysterious than most biological problems; for the modern man of science is little more able than the simple-minded savage or the poet or prophet to tell how all the co-ordinated faculties of a predaceous animal are so thrown into action by the stimulus of hunger as to lead to the pursuit and capture of prey; yet there is no mystery in the physiology of hunger, for, while there is much we do not understand, we do know that hunger incites actions which are responsive, or adapted for satisfying hunger.
So also it may be possible to make progress in the study of the meaning of migration in spite of our ignorance of the nature of the impulse which excites and determines it; and while I gratefully acknowledge my debt to Newton for the facts, I am not able to agree with him that there is anything distinctively or peculiarly mysterious in the subject.
While there is reason to believe almost every bird of temperate and arctic climates is migratory to some degree, those which simply range over a wider area at one season than at another present nothing notable, and it is only in regions which are almost or quite deserted by birds for part of the year that their migrations attract the attention of students. As many birds which are most valued for food are found in temperate regions for only a short time in the spring and fall, sportsmen and hunters and all who pursue them for food have been familiar with the habits of the birds of passage from the dawn of history; but most of the best literature on the subject is by northern ornithologists, and the home of the writer has had and still has great influence on opinion as to the meaning and origin of the migratory habit. Scandinavians and Saxons and Anglo-Saxons are home-loving folk, who, in all their wanderings through this world of care, keep a warm affection for the fatherland.
A learned professor in the University of Upsala once wrote a book to prove that the garden of Eden was in Sweden, by the simple argument that no one who knows the delights of that blessed country can believe paradise could have been anywhere else. He showed that the Atlantis of Plato, the country of the hyperboreans, the garden of the Hesperides, the Fortunate Islands, and the Elysian Fields are but faint and imperfect reminiscences of the lovely and favored climes of Sweden, from which the Greeks themselves derived their alphabet, their astronomy, and their religion.
To men of the north, home seems the natural refuge of the birds, and, as much of the literature of migration is northern, the birthplaces of common birds have been regarded as their true or natural homes, and while their disappearance in winter has seemed to call for explanation, their return in summer has been looked at as a matter of course, for the intense love of home which many birds have has seemed enough to draw them back as soon as winter is over. It is the "homing" instinct which makes the carrier pigeon so useful to man; and one of the most impressive features of the migratory habit is the definiteness of the journey northward, which often ends in a particular bush or ledge of rocks. Many of our common birds lay their eggs year after year in the same nest, although they spend part of the year in the heart of a foreign country thousands of miles away, and although the surroundings of the chosen spot may have changed so much that it is no longer a judicious selection. A bottle in the branches of a tree at Oxbridge, in England, is known to have been occupied every year, with only one exception, since 1785, by a pair of blue titmice; and on a hill in Finland, well known to tourists as the most northern point in Europe where the sun can be seen at midnight, a nest is said to have been occupied by a pair of peregrine falcons every year since 1736. Many like cases are recorded, and while it is not probable that the birds which visit a nest year after year for centuries are the same, the fact is all the more remarkable if they belong to successive generations.
According to folklore, some of the summer birds hide near home through the winter, and Cams, in his History of Zoölogy, refers to several learned writers who, early in the seventeenth century, quoted from the older literature much venerable authority for the belief that swallows hide through the winter in holes and in clefts in the rocks, and even under the water.
Many writers on migration have believed, as they have been taught from childhood, that the birds go south to escape the rigors of a northern winter, although little reflection is needed to show that no animals are better protected or more indifferent to changes of temperature, or that, while sea birds are highly migratory, the open waters of arctic seas are little colder in winter than in summer. Nestlings are often killed by exposure, and eggs require a high external temperature, but old birds are, as a rule, indifferent to cold.
When this is recognized, the prevailing belief is that birds leave their homes in search of food, for scarcity is most assuredly an important factor in the origin of migration; but this view of the matter fails to show why, with the whole world to choose from, they do not settle in lands which are habitable the year round.
"The shuddering tenant of the frigid zone
Boldly proclaims the happiest spot his own,"
and the return of the birds seems only natural to the Eskimos; but to us who are not Eskimos the wonder is not that anything which can get away should do so, but why the birds pass by so many lovely and fertile regions to seek a home in the barren and desolate ends of the earth; and it is plain that, of the two journeys which make up the migration, the summer visit to northern lands and waters is at least as remarkable and as well worthy of consideration as the journey southward in the fall.
Failure of food in their birthplace is no doubt the chief reason why the migratory birds do not spend the whole year there, and in so far is an explanation of migration, for no animals are better fitted for moving from regions of scarcity to regions of abundance, although they are no more able than creeping things to establish themselves in new lands which are already well stocked with inhabitants; for they are kept within the limits of their natural habitat, like other animals, by competitors and enemies, rather than by physical barriers, although their power to wander and to overcome physical barriers is without a parallel. There are few oceanic islands, however remote, which are not inhabited by land birds descended from lost wanderers, who, finding these spots unoccupied, have been able to establish themselves. The list of North American birds which are occasionally found in Europe is a long one, and stray specimens of the gray plover, whose summer home is the shore of the Arctic Ocean, have been found at the Cape of Good Hope, in Ceylon, in Australia, in New Zealand, and in Tasmania. Most of the wanderers are shore birds which make long migrations, and being much exposed to storms are often driven far out of their path; but this is not always the case, for the great albatross follows ships across the whole breadth of the South Pacific, or nearly half the circumference of the earth. Many birds seem to make their whole journey by a single flight, for some which are common in the West Indies and in Nova Scotia are almost unknown within the limits of the United States, making the whole journey past our borders by water and probably by a single flight. The bluethroat, which breeds in the northern part of Scandinavia, is so seldom found in Europe south of the Baltic that there seems to be good evidence that it makes its whole journey to its winter quarters in the region of the upper Nile by a single flight.
There is no reason to suppose all migratory birds inherit this habit from a common ancestor, nor that its purpose is always the same, and many birds of prey seem to have acquired it by ranging far in winter in search of food, and by following their prey into warmer regions, to return to their birthplace in the breeding season.
In those cases the birthplace may have been the original home, before the migratory habit was acquired, and scarcity of food the reason why it was acquired; and 'the influence of scarcity in causing migration is well shown by the occasional or irregular migrations of certain prolific animals which do not ordinarily leave their birthplaces, although, when these become overstocked, migrations take place, just as human colonists go out from thickly settled countries to find new room for growth in foreign lands. From time to time, at irregular intervals, great armies of the smaller and more prolific rodents, which usually spend their lives where they are born, are met on the march from homes where overproduction has exhausted the food; and several of the older American naturalists have described the migrations of our gray squirrel, although the phenomenon has been most carefully studied in the Norwegian lemming, whose remarkable migrations have figured in literature for centuries. The lemming is a small, restless, pugnacious, and very prolific rodent, which at uncertain and irregular intervals of from five to twenty years migrates from its home in the central mountain chain of Norway, and invades the low lands so suddenly and in such numbers that it is still popularly believed, as in the day of Olaus Magnus, who wrote in 1490, to drop from the sky.
The great army of lemmings travels in a straight line and overruns the cultivated country, swimming the lakes and rivers, and causing so much destruction that a special formula to be employed against it was at one time authorized by the Church, which attempted to check its march by exorcism, just as the old Bishop of Montreal tried to drive away the wild pigeons by anathemas. The lemmings travel at night, but their march is not continuous, for they make long halts in fertile spots, where they are even more prolific than they were at home, so that they become more and more numerous, although they are attended by bears, wolves, foxes, dogs, and cats, and by hawks and owls, and other beasts and birds of prey, and although even the cattle and reindeer are said to kill and eat them. The march may last for several years, but as they never go back, but continue to move forward, they at last reach the ocean, and, attempting to swim this as they have all the rivers in their course, all are drowned.
While the migration of the lemmings is undoubtedly due to scarcity, it is difficult to understand its use, for at the present day the only ones to profit by it are those who have the instinct least developed and stay at home in the mountains, although it may have been useful to the species before the low lands were occupied by man, who now destroys the stragglers and prevents them from scattering and finding permanent homes.
While the determining influence is the scarcity which comes from overcrowding, we have no reason to believe the lemmings consciously and deliberately set out to find a better feeding ground, or that they have traditions of the rich low lands which attract them as the wealth and luxury of China and Mesopotamia and of the Roman Empire attracted the Tartars and Scythians and Goths from the sterile and desolate northern lands into the fertile homes of southern civilization.
Their journeys are no doubt initiated by an unconscious impulse, which, before it brought them into conflict with man, was useful in some way to the species; and this seems to be true of the migrations of certain prolific species of locusts and grasshoppers, which, inhabiting sandy deserts, often overflow the limits of their natural home, and invade more fertile regions where they are not usually found. While there is no reason to suppose these movements are undertaken through deliberate intention to find new feeding grounds, lack of food is no doubt the chief factor in the development of the migratory instinct of rodents as well as locusts, which latter resemble birds in ability to make long journeys on the wing without rest. The African locust has been met at sea in great clouds more than twelve hundred miles from land, and this species sometimes wanders from its home in Africa to England.
While the movements of rodents and locusts show that the search for food has much to do with migration, they lack the features which make the migrations of birds so remarkable. They occur at irregular intervals, while the movements of birds are almost as regular as the almanac, for, while sea birds seem much exposed to storms, the days of their arrival and departure can be predicted as if they were satellites revolving round the earth. "Foul weather or fair, heat or cold, the puffins make their appearance at the proper day as promptly as if they were moved by clockwork." While the course of the migration of rodents and locusts is determined by conditions so complicated and irregular that they may be called accidental, the northward journey of birds is often directed to a definite spot thousands of miles away from the starting point, and the resemblance between irregular migration in search of food and the migration of birds is too imperfect to tell us much about the origin of the latter, which resembles more the movements of fishes like the shad, which at a definite season enters upon a journey along a definite path to a spot hundreds of miles away, to return again after the purpose of the journey is accomplished.
Since the number of shad which enter a river in the spring is out of all proportion to its resources as a feeding ground, we might say of them, as we are disposed to say of birds, that they leave their birthplace in search of food; but as they find so little food in the rivers that it may be said, with almost literal exactness, that they make their journey fasting, it is quite plain that this is the wrong point of view; that we must believe they enter the river to lay their eggs, and that we must see in this, and not in the return to the ocean, the purpose of the migration.
As the shad is a marine fish which does its eating at sea, and as its visits to fresh water are only for the purpose of reproduction, the numbers which make their way up the rivers are out of all proportion to the capacity of the streams for supplying them with food. The shad enters the mouths of our rivers in the spring in great schools, and travels up them to a most surprising distance; for the total length of the journey from the sea to the spawning ground and back again often exceeds a thousand miles, and this journey is made almost or quite without food. Many of them, and among these the largest fishes, go on until they meet some insurmountable obstacle, such as a waterfall or a dam, or until they reach the head waters of the river. Before dams were built in the Susquehanna, many shad which entered the Chesapeake Bay at the Capes continued their long-fasting journey across Virginia, Maryland, and Pennsylvania into the State of New York, and traveled through more than five hundred miles of inland waters on the journey upward.
Fragments of Indian pottery, stamped with a pattern made by the impression of a shad's backbone, have been found in southern New York, and the number of stone net-sinkers which have been picked up in the Wyoming Valley shows that the Indians had known and used these fisheries long before the first white settlers found them at work with their rude seines. In the early part of this century, before canals and the dams which supply them were made, there were forty fishing stations beyond the forks of the Susquehanna in northern Pennsylvania, and some of them were worth from one thousand to twelve hundred dollars a year to their owners. There is a record of the capture, at a single haul, of ten thousand shad at one of these fisheries on Fire Island, near Wilkesbarre. Dams across the river have excluded the shad from more than two hundred miles of the course of the Susquehanna, and the profitable fisheries now reach for only a few miles above the boundaries of Maryland, while the shad are cut off from many of the best breeding grounds, which are the sandy flats near the shores of streams and the sand bars which lie in their course. The fishes run up into these places in pairs in the early evening after sunset, and the eggs are thrown into the water while the fishes are swimming about, but they soon sink to the bottom and develop very rapidly. The number of eggs is about twenty-five thousand, but a hundred thousand have been obtained from a single large shad. Few adult shad escape all the dangers of their journey, and these few are so battered and emaciated that they have no value as food, and are unknown in our markets, which are supplied with those that are captured on their way upward. The young fish remain in the rivers until late in the fall, feeding upon small crustacea, the larvæ of insects, the young of other fishes, and minute active animals, and they grow to a length of two or three inches by November, when they leave our waters for the ocean. The shad is a marine fish which has acquired the habit of laying its eggs in fresh water, out of reach of the innumerable enemies that abound on the shoals and sand bars of the seashore. Since the eggs are abandoned by their parents soon after they are laid, prolonged residence at the breeding grounds is not necessary, and the shad has thus been able to utilize safe places which supply no proper food and are unfit for prolonged residence. If it were compelled to incubate its eggs and to guard and protect and feed its nestlings like a bird, it would have been restricted to some breeding place fitted for more prolonged residence, and we should then feel something of the same tendency to call its birthplace its true home that we experience in our study of birds. We should refer the migration to this place as the starting point, and we should try to discover some reason why they spend part of the year elsewhere.
Most animals owe their existence to the occurrence, in their natural home, of all that their life requires, but the power to traverse great distances at great speed, and to pass over all the barriers of land and water, joined to their comparative indifference to changes of temperature, permits birds to divide their time between widely separated regions, and, whether the choice be conscious or unconscious, the breeding places of migratory birds are selected on account of their safety, and not because they furnish all that a permanent home must supply. If we believe, with Professor Marsh, that the power of flight was acquired by birds after they became arboreal, we must look for the primitive home of the migratory birds in the great tropical and subtropical forests where arboreal reptiles and arboreal mammals still abound; nor can we believe the great armies of northern birds which find abundant food in southern lands in winter are driven out by scarcity on the approach of spring. Enemies are numerous in the tropics, but no animals have sharper senses or better means of escape than birds, and, trusting in their power of flight and their quick sight and hearing, they venture into danger with confidence. The great charm of birds to us is the fearlessness with which they approach man, who is the most dreaded enemy of all other vertebrates; but while adult birds are eminently fitted for taking care of themselves, the opposite is true, in even greater degree, of nestlings, for no animals are at the same time more helpless and more exposed to danger than many young birds, while eggs are not only absolutely helpless but also very tempting to enemies, although there is no group of animals in which the safety of the eggs and young is more important. Among birds a high birth rate is incompatible with flight, for their eggs are large and heavy, and the preservation of each species imperatively demands that every egg shall be cared for with unceasing solicitude; for while, in other animals, increased danger to eggs and young may be met by an increase in the birth rate, this can not be much increased in birds without corresponding loss in the power of flight. Every one knows how quickly birds are exterminated by the destruction of their eggs and young, and the low birth rate of all birds of powerful flight is a sufficient reason for migration, for at the same time that flight limits the birth rate it permits the birds to seek nesting places beyond the reach of their enemies; and as there is rigorous selection of the nestling's which are born in safe nests, it is easy to understand how the instinct has been gradually fixed by selection, and how, as it has become more and more firmly fixed, and as the safety of the eggs and young has become assured by the remoteness and isolation of the nests, the birth rate has been still more reduced and the power of flight correspondingly extended. Many sea birds that nest on desolate rocks in midocean lay only one egg each year, and have the power of flight in its highest perfection. The power of the storm petrel to wander is as boundless as the ocean, and while it lays only a single egg, it is said to be the most prolific of birds, since the number of individuals is greater than in any other genus.
We can not believe that all migratory birds inherit the habit from a common parent which was migratory, nor is it probable that in all cases it owes its origin to the same influences; but if the view which is here advanced be correct, we must believe that in most migratory birds it has been brought into existence by the needs which are involved in reproduction, and not by the supply of food, and that the winter home of birds in tropical and temperate regions, and not the birthplace of modern birds, must be regarded as the starting point for the migratory habit.
While Wallace was the first to recognize the importance of selection in the formation of this and other instincts, he seems to think selection alone, without the influence of geological change, is inadequate to explain all the facts of migration. He says: "It appears to me probable that here, as in so many other cases, 'survival of the fittest' will be found to have had a powerful influence. Let us suppose that in any species of migratory birds breeding can, as a rule, be only safely accomplished in a given area; and, further, that during the great part of the rest of the year sufficient food can not be obtained in that area. It will follow that those birds which do not leave the breeding area at the proper season will suffer, and ultimately become extinct, which will also be the fate of those which do not leave the feeding area at the proper time. Now, if we suppose that the two areas were for some remote ancestor of the existing species coincident, but by geological or climatic changes gradually diverged from each other, we can easily understand how the habit of incipient and partial migration at the proper seasons would at last become hereditary, and so fixed as to become what we term an instinct. It will probably be found that every gradation still exists in various parts of the world, from a complete coincidence to a complete separation of the breeding and the subsistence areas; and when the natural history of a sufficient number of species is thoroughly worked out, we may find every link between species which never have a restricted area where they breed and live the whole year round to others in which the two areas are absolutely separated."
Modern zoölogy owes its scientific basis to the work of Wallace and Darwin on the distribution of birds, which, in their hands, has led to a revolution in our conceptions of Nature, and has given so much weight to their opinions that no one would venture to differ from them inconsiderately, although when we try to interpret, in the light of his other writings, Wallace's assertion that "the habit of incipient and partial migration" may "at last become hereditary," we must doubt whether he has carefully weighed his words.
We must also remember that distribution and migration are distinct phenomena, and that while the geographical distribution of birds shows clear indications of the effect of past geological changes in the distribution of land and water, migratory birds are kept, like other birds, from invading other provinces than their own by competitors and enemies rather than by geographical barriers.
As so many birds move toward the poles of the earth to lay their eggs, and toward the equator to spend the winter, the view that their two homes have been drawn apart by changes of climate seems probable at first sight, but the rule is not universal, for many of the great breeding grounds of sea birds are in temperate or tropical regions. The petrels, albatrosses, terns, gulls, and many other birds pass most of their life scattered over the ocean, but this affords no nesting place, while the wastes of water which keep carnivorous mammals and reptiles and other enemies of nestling birds from the remote and desolate rocks and sand bars of the open ocean are no obstacle to them. These spots are so secure that birds born in them are much more likely to survive than those born on the shores of inhabited lands, so that it has come about that all or nearly all of the modern members of these groups are descended from ancestors which shunned the dangerous nesting places, not because acquired habits have become hereditary, nor because their feeding ground and their nesting place have been drawn apart by geological change, but because all which did not instinctively lay in safe places the few eggs which are all their fitness for continuous and rapid flight permits, have been exterminated. These birds now gather from all parts of the ocean on the few widely scattered rocks and islands where their young are safe, and the periodic assemblies of innumerable multitudes of wandering sea birds in the "rookeries" are true migrations, for they are as regular as the almanac in the time of arrival and departure, although their feeding ground is almost as extensive as the ocean and the food supply has nothing to do with their movements, and although they do not reach the "rookeries" by a single path.
In this case the needs of reproduction are the controlling influence, and the site of the "rookery" has been fixed by its safety; and while it is difficult to say how far the birds are guided by knowledge of the danger of other places, the well-known tameness of sea birds in their breeding places, and their apparent ignorance of the existence of enemies seem to show that they are quite unconscious of the advantages of the chosen spot, and that they resort to it automatically or naturally not because they know its safety, but because they owe their survival and existence to the fact that it is safe.
Zoölogists are far too ready to resort to the boundless fields for speculation which geology affords, and it has been gravely suggested that the migration of the lemmings and their death in the waters of the ocean may be due to their efforts to reach the lost Atlantis, where their ancestors dwelt during the Miocene period, although this opinion has no better basis than the belief of Olaus Magnus that they rain down from the sky, where they are engendered from the decomposing exhalations from the clouds impregnated by the semen of rats.
It is easy to understand how birds near the northern limit of their range invade the territory of those whose home is a little farther south, and compete with them for food as this becomes scarce with the approach of winter, and how this movement spreads until all the members of the species are involved, although many of them might have been able to subsist some time longer in their breeding ground if they had been undisturbed.
We have seen that this has commended itself to northern naturalists as a sufficient reason for the acquisition of the migratory habit, and the fondness for their birthplace which is so strongly marked in birds has been thought enough to draw them back; but love of home is itself a result of natural selection, and the necessity for finding safe places for the eggs and young is enough to account for the migration without the aid of geological changes.
While we know little as to the means by which birds find their way over land and water, we know that, as a matter of fact, they are able to do so; and we also know that the instinct which leads them to seek safe places for their nests is so firmly implanted in their nature that centuries of domestication weaken it but little, for it is still almost as strong in the Guinea fowl and the turkey and the hen as it is in wild birds. As birds of powerful flight have a range of choice almost as wide as the earth itself, it is not surprising that the continual destruction of those born in the least safe nests has at last resulted in the survival of the ones which make their nests thousands of miles away from their natural or ancestral home.
While most writers have thought migration had its origin in an annual journey which, while short, was definitely fixed for all the members of the species, and while they have felt forced to call in the aid of geology to account for the gradual separation of the two termini and the lengthening of the journey, the hypothesis of geological change seems gratuitous and unnecessary, since the known habits and instincts and needs of the birds are in themselves a sufficient explanation of all the broader and more general characteristics of migration. It seems much more simple, and more consistent with our knowledge of the past history of living things in general, to believe it had its origin in an intense but geographically indefinite impulse which led birds to scatter at the breeding season and to hunt out safe hiding places for their nests; and that, as enemies also improved in power to find the most accessible nests, the instinct has been gradually shaped into definiteness by extermination and natural selection, until at last safe breeding grounds, far away from home and far out of the reach of natural enemies, have become established, and until many species and all the members of each species have come to share the impulse to resort to the selected breeding places on the approach of sexual excitement, and to follow the same path between distant points; that the increasing safety of the eggs and young has permitted a low birth rate and the improvement by selection of the power of rapid and long-continued flight; and that this has in its turn permitted the migration to become longer and longer, and more and more protection to the eggs and young.
The history of migratory birds has been long and complicated, and there has been time for great changes in climate and in the distribution of land and water, and these have no doubt left some permanent impression on the habits of birds. The birds have not eluded all their enemies, for predaceous birds and their prey are found together at both ends of the journey. New ways to escape enemies and new ways to find food are as important as they ever were, and the details of the subject are very complicated, although it seems clear that its broader outlines admit of explanation without recourse to geological changes or the inheritance of the direct effects of the conditions of life.
In conclusion, I wish to remind the reader that our present interest in migration lies in its value and simplicity as an illustration of the general law that the adaptations of Nature are for the good of the species and not for the benefit of the individual.
This law is universal, but since the welfare of the species is usually identical with that of the constituent individuals, it is not obvious unless the good of the species demands the sacrifice of individuals.
Long journeys are hazardous. Every California salmon which enters on the long journey to the breeding ground is destroyed, and the whole race of adult fishes is wiped out of existence, for the good of generations yet unborn. Few shad ever return to the ocean, and storm and accident and ruthless enemies work their will on the migrating birds and decimate them without mercy, although the dangerous return to the safe breeding grounds still keeps up, in order that children which are yet unborn may survive to produce children in their turn.
The safeguards which Nature throws around eggs and infants and the immature, and the indifference to the fate of the mature animals which is exhibited by the influences that have modified species into fitness for their environment, are facts which must never be lost sight of, for if we forget them our attempts to understand the history of the properties of living things are certain to mislead us and to end in failure.