ACCLIMATIZATION, in the broad sense, is a term applied to the fact that plants or animals may be introduced, artificially or naturally, into a country with a climate different from that of their original habitat, and may successfully propagate there. What would come to the same thing is the successful persistence and propagation of plants and animals in a country whose climate has undergone some notable change, such as would be involved in the setting in of an ice age or a period of aridity. In this broad usage the term acclimatization is practically synonymous with naturalization.
But in its narrow usage acclimatization means a process by which organisms are changed in the course of generations in adaptation to new climatic conditions in the same or in another country. This usage implies the theoretical postulate of racial adaptation to withstand climatic influences which were at first more or less unfavourable. The establishment of the adaptation might be interpreted by some along Lamarckian and by others along Darwinian lines, and this raises additional theoretical questions.
Thus some investigators, e.g., G. M. Thomson, who find little evidence of acclimatization in the strict sense, prefer to keep the term naturalization, which expresses an indisputable fact that organisms may thrive well when taken to another and in some respects different country. Willis defines acclimatization as "the accustoming of plants to new conditions and climates till they are not only capable of growing there but also of reproducing them selves freely." Thus, though the cherry and apple will grow well in the hills of Ceylon they are not acclimatized, for they do not produce fertile seed.
(2) Another consequence that has been repeatedly noted is increase in individual size and perhaps vigour. The new conditions prove unusually stimulating. Speaking of plants introduced into New Zealand, G. M. Thomson writes : "Water-cress—a plant of two to four feet in length in European waters—grew in some streams to a length of from 12 to i4ft., and with stems as thick as one's wrist." This riotous exuberance is due positively to the stimulating conditions of the new territory, and negatively to the absence of the previous checks.
The popular interpretation of the rapid spread of introduced plants, e.g., "weeds," in new countries, is that they come from places where the struggle for existence is keener and where they have therefore become particularly efficient. According to Willis, "the real explanation, in all but a very few doubtful cases, is that their spread is due to change of conditions. This has usually been effected by man, who has often altered, or even destroyed, the conditions under which many societies of plants formerly flour ished, thus giving a fair field to those newcomers that were suited to the new circumstances." (3) Many naturalists have concluded that the lessening of the stringency of natural selection after transport to a new country allows an increase in the number of varieties, and the survival of peculiar forms, which would be speedily eliminated in the original environment. Thus, if there are few enemies, one might expect more numerous conspicuous variants, such as albinos. Here the careful work of G. M. Thomson is of great value. In 1891, he concluded that conspicuous colour-variations were on the increase among the rabbits introduced into New Zealand, and also among introduced birds like sparrows, thrushes, blackbirds, skylarks and starlings. Thirty years later he definitely withdrew this conclusion, the fallacy being that he was at first so busy looking for anomalous characters that he met with many, and unconsciously exaggerated the ratio of their occurrence. But inquiry must be made in other fields to see whether there is nothing to be said for the old view.
(4) So many naturalizations have been effected in different parts of the world that one would expect to find it easy to collect instances of change of habit ; but there seems to be a strong con servative tendency among 'animals introduced into new haunts. When domesticated animals are transported to a wilder country there is sometimes an interesting individual rehabilitation of a long lost ancestral trait. Thus cows taken from Scotland to wilder conditions in California have been known to hide their calves in the thicket when they went to graze in the open. Some times, however, something novel occurs. Thus, sheep in New Zealand have shown in some places the novel habit of stripping off long pieces of bark from the gaya trees. On the whole it seems that change of habit in consequence of naturalization is infrequent and very gradual.
(5) Some types are quickly at home in a new country, but do not show external change. Horses, rabbits, rats, sparrows, and fowls are usually somewhat indifferent to change of climate, while a tough animal like the yak of the Tibetan mountains refuses to thrive below a certain altitude.
Individual adjustments may be of much importance though they do not meet the eye, as in the case of adaptation to high altitudes. When a man accustomed to life near sea-level settles in a place at a high altitude, there is likely to be, if he thrives, an increase in the haemoglobin content of the blood. This is due at first to a reduction in the volume of the plasma and a concentration of corpuscles, but secondarily to an increased cor puscle-formation. At very high altitudes there is some evidence of an increase in the affinity of haemoglobin for oxygen. Another change is demonstrable in the reaction of the blood to the acid side, and this is also useful since less carbon dioxide is required to stimulate the respiratory centre, thus increasing pulmonary ventilation. We have referred to this adaptation to high altitudes because it illustrates an adjustment that can be established only by intimate inquiry. It is highly probable that similar adjustments are exhibited by many animals when they have to face a marked climatic change.
Many facts confirm the suggestion that the success or f ailure of attempted naturalization may depend on inconspicuous con stitutional peculiarities. Thus G. M. Thomson notes for New Zealand that the greenfinch and the chaffinch have thriven re markably, while the allied linnet has failed. "The reasons for these failures are often so obscure that no plausible explanation has yet been given." A hint of the frequent subtlety of conditions may be found in cases where the attempted naturalization of a plant fails, as of heather in Ceylon, because the associated root-fungus or mycor rhiza will not grow.
(6) When an organismal change directly induced by some change in environment, nutrition, or habit, takes such a grip that it persists after the inducing conditions have ceased to operate, it is called a modification, or less conveniently, an individually acquired character. There seem to be some climatic modifications, and the following may be mentioned : (a) An Englishman who works half his lifetime under a tropical sun may become so tanned that the deposit of melanin pigment in the skin does not disappear during all the years in which he enjoys his pension at home. Of course it must not be inferred that the blackness of the negro's skin was directly produced in this way by the tropical sun. (b) Nageli brought some Alpine plants to the Botanical Garden at Munich, and there many of them became in the first year so much modified that they were hardly recognizable as the same species. Their descendants in the garden were also quite different from the Alpine originals. Thus the small hawkweeds (Hieracium) became large and thickly branching with abundant blossoms. The modi fications were very striking, and in some cases many generations were observed—even for 13 years. The re-appearance was prob ably due to the fact that the original modifications were directly re-impressed on each successive crop, for when the plants were removed from the rich garden to poor, gravelly soil, the acquired characters gradually disappeared, and the plants exhibited once more the original Alpine characters. There was no convincing evidence of hereditary entailment.
It is important not to think of these matters too simply. Thus, as Goldschmidt points out, the normal development of particular characters, such as general growth and pigment formation, has been "harmonized" for a definite environment. But they have different temperature coefficients, and novel conditions may throw them out of harmony. Moreover, modifications resulting from climatic change must not be thought of as necessarily beneficial. Thus some Lepidoptera tend to melanism in the cold, and others at high temperatures, without there being any demonstrable ad vantage in either case.
(7) If evidence could be obtained of the hereditary entail ment of climatic modifications, this would serve as a basis for a Lamarckian theory of acclimatization. It is necessary to inquire afresh whether there are any facts supporting the Lamarckian interpretation. Bordage records some observations on peach-trees (Prunus persica) grown from seed of European origin sown in Reunion. For ten years or so the trees shed their leaves as in Europe. Later on, after zo years, a considerable degree of ever greenness was exhibited. There was no bare period. When seeds of these partial evergreens were sown in the lowlands they grew into trees verdant throughout the year, but the same was true of seeds sown in certain mountainous districts with a considerable degree of frost. They also grew up into young peach-trees which were also evergreen. But European seeds sown in similar situ ations developed into ordinary deciduous trees. This is a peculiar case and may be interpreted as follows : The original imports underwent a gradual constitutional change—some modification of their metabolism ; this might affect the constitution of the seed during the period when it was still part and parcel of the parent's body. The change in the metabolism might result in the affected seeds developing into evergreens, though the direct environmental influence would work in the opposite direction. It is unfortunate that the observations were not carried f urther.
A famous case, often referred to, is that of the feral horses of the Falkland islands, which Darwin studied on his "Beagle" voyage (1833). He says that the horses and also the cattle were intro duced by the French in 1764. Whereas the cattle are large, the horses are small, and Darwin speaks of them as "having de generated." "They have lost so much strength that they are unfit to be used in taking wild cattle with the Lazo : in consequence, it is necessary to go to the great expense of importing fresh horses from the Plata. At some future period the southern hemisphere probably will have its breed of Falkland ponies, as the northern has its Shetland breed." Darwin regarded the degeneration of the Falkland islands horse as due to the humid climate and the lack of suitable food, but it is difficult to believe that marked changes were effected between 1764 and 1833. It would be necessary to know more about the horses originally imported.
Various statements have been made in regard to changes brought about in the hair of sheep, goats, cattle, sheep-dogs, and even cats, when these animals are taken to a more rigorous climate. It is said that the fur becomes longer and thicker, which would be a useful adaptation. But there is a lack of precision in these statements, and a regrettable absence of measurements. It would be useful to know what additions were made to the coat after the climatic change ; whether the offspring, exposed to the cold from birth, showed further additions; and the character of the fur in the grand-offspring. As regards modifications, no in ference can be drawn from the occurrence of adaptive peculiarities, unless the history is known.
On the whole we are forced to the conclusion that the evidence of the heritability of climatic modifications is unsatisfactory.
Three saving-clauses should be kept in mind: (a) When two nearly related species are thriving in climatically different sur roundings, it should not be taken for granted, as it usually is, that all their differences are now part and parcel of the inheritance. Some of the differences may be modificational, hammered on each successive generation in the course of development. There is need for more experimental study of species. (b) There has been a tendency to strain the interpretation of specific characters as adaptive to particular conditions of life, such as those implied in climate. Many characteristics separating related species in different localities may be reasonably interpreted as climatic adaptations, but each case should be carefully judged on its merits. (c) When a species is extending its range in consequence perhaps of increasing numbers, the factor of isolation may come into oper ation, say in the form of a river or a watershed, and variations may be separated off which have no particular relation to the new territory or climate in which the leaders of the advance find themselves. Thus new species may arise by the physical segre gation of diversely varying contingents of an advancing army, till the climatic difference itself may become an isolating factor.
Accepting, with these saving-clauses, the idea of natural accli matization, we must now ask how it may have been effected. The Lamarckian answer involves the postulate of the transmissibility of modifications, especially of functional modifications; and the evidence of this, summed up by Kammerer (1924) and by MacBride (1924), appears inconclusive. The interpretation which seems to involve fewest assumptions is that of Darwin. In dis cussing acclimatization (1868), Darwin laid most emphasis on the natural selection of spontaneous variations. As to these vari ations he expressly says "there is no evidence that a change in the constitution of the offspring necessarily stands in any direct relation with the nature of the climate inhabited by the parents." In regard to selection he lays emphasis on two points : (a) the organism's power of resistance to difficult conditions in the new climate, and (b) some useful change in the period of reproduc tion, such as earlier flowering and fruiting.
To Darwin's interpretation an addition may be suggested. It is conceivable that the climatic peculiarities may affect the meta bolism of the organism through and through, and may thus serve as stimuli to the variability of the germ-cells. If the climatic peculiarity should induce (a) an adaptive modification in the body of the organism and (b), at the same time, a variation in the germ-cells which finds expression as a similar new character in the offspring, the phenomenon is called "parallel induction." It must be distinguished theoretically from the transmission of a somatic modification ; it is a change induced in the germ-cells along with, but not through, the bodily modification.
But it is possible that climatic peculiarities may penetrate into the germ-cells and affect them without producing any modification in the body. Thus W. L. Tower subjected full-grown potato beetles (Leptinotarsa) to peculiar conditions of temperature and humidity during the time when the eggs were maturing, and found that "mutations" occurred in a certain proportion of the offspring. The parents were not affected, having passed the plastic stage; and some of the eggs were not affected at all. Moreover, the same environmental peculiarity, analogous to a climatic change, did not always evoke the same mutation. Some of the mutations in colour and markings were very striking; others affected minute details of structure. In subsequent generations there was no reversion. This case is of particular interest in connection with acclimatization, for the artificial environmental conditions, effected in large steel and glass cages, were compar able to different climatic conditions in which different species of potato-beetle live.
By Muller in particular it has been shown that an exposure of the germ-cells of the fruit-fly (Drosophila melanogaster) to appropriate doses of X-rays is followed by numerous mutations, which often breed true. This experimental irradiation illustrates the possible action of the environment as a variational stimulus. A mutation following a climatic change might have this origin. As Weismann said many years ago (1904) : "It does not seem impos sible that the climate may have a variational influence upon certain determinants of the germ-plasm, for we have already seen that the influence of cultivation may incite plants and animals to hereditary variations, and that slowly increasing disturbances in the equili brium of the determinant system may thereby be produced, which may suddenly find marked expression as 'mutations.' But there is little probability that adaptations, that is, transformations cor responding to the altered climate, can arise in this way!" The meaning of this last sentence is that Weismann believed these adaptations were the outcome of the prolonged natural selection of fluctuating variations in the germ-plasm.
The Darwinian theory, as applied to natural acclimatization, may be briefly resumed in more technical terms. In any species in any given environment there will be an observable percentage of variations from type in a given number of births, and these variations yield a curve of freque ncy or probable error. The steepness of the curve is a measure of the variability of the species. The aberrants on all sides are pruned off by selection. If the environment is changed, selection may no longer operate on the same axis as before, but may tend to prune off variations on one side of the mean more than on another. In the course of time, the apex of the curve, representing the type form, will shift to suit the new conditions, since more of the aberrants on the favoured side will live to reproduce.
Quite a separate question is this : that the new environment may increase the variability of the species, flattening the curve of probable error. The effect of this is that selection has more material to work with, and therefore attains its end more rapidly, although of the variations appearing as many will be unfavourable as favourable. A species in which there is little variation presents a greater inertia to the shifting effect of altered climatic conditions than a variable species. In the less variable species there will be relatively fewer favourable variations to mate with the type.
Bateson's Silliman Lectures contain a valuable criticism of the somewhat facilely accepted views (a) that local and climatic varie ties are adaptational, (b) that the influences of environment have directly led either to the production of these varieties or to their selective stabilization, and (c) that there is gradual transition—or mass transformation—from one species to another in response to climatic influences. Bateson lays much emphasis on the role of isolation and on the intrinsic character, e.g., Mendelian domi nance of the sporadic variations that are of frequent occurrence.
Of importance in connection with acclimatization is Erwin Baur's study of cultivated snapdragons, especially Antirrhinum majus. He finds a frequent occurrence of small mutations, trans missible in Mendelian fashion, often showing themselves in "pure lines," and sometimes suggesting an enhancement of vigour, as when a mutant appears with deeper green in its leaves. According to Baur wild species of snapdragon, like garden races, have often arisen by the summation of small mutations.
(3) Some animals able to move about for considerable distances and not too slowly would be prompted by climatic changes to shift their quarters. As the severity of the ice ages spread southwards in Europe, many northern mammals came with it, thus remains of reindeer, lemming, and arctic fox are found in deposits far to the south. As milder climates set in and the glaciers melted, the descendants of some of the arctic types, like reindeer and white fox, were able to trek for the north. Some, however, remained as refugees on the mountains, like the snow vole (Microtus nivalis) of the high Alps. Some of the true bird migrations may owe their origin in part to distant climatic changes such as those of the quaternary ice ages (see MIGRATION).
(4) It is reasonable to suppose, though difficult to prove, that change of climate in a country induced important changes of habit. Thus Barrell and Lull have suggested that continental ele vation and consequent aridity, especially in the Himalayan region, led in the Miocene or early Pliocene ages to a dwindling of the forested area where man's ancestors were at home. The alter natives were to find other forests in warmer countries, as the present-day anthropoid apes did, or to be eliminated, or to come to earth, and begin afresh on a new line of life. The last solution may have been of critical moment in the evolution of Hominoids.
(5) Climatic changes in a country may also have played an important part in punctuating the life-history. A kind of variation which has not received adequate attention may be called "tem poral." It includes alterations in the tempo, or rate, or rhythm of metabolic processes, or in the duration of particular phases in the life-history. In vertebrate animals, at least, this might be brought about by variations (also, of course, requiring to be accounted for somehow) in the secretory activity of the ductless or endocrinal glands, the hormones of which serve now as accel erators and again as brakes. The life-histories of many types differ from one another in the shortening or lengthening of par ticular arcs on the life-curve or trajectory. Here is a kind of evolution to which climatic variations may have applied a frequent spur. Thus when the rate of development was such that the lif e cycle could not be completed in the first summer, there would be a tendency to favour variations in the direction of interpolat ing a larval phase, as in insects, suited for an accumulation of reserves, a reduced intensity of life in cold weather, a diminished exposure of vulnerable surface, and so on. Opinions may differ in regard to particular cases, but it is a legitimate and instructive inquiry to associate temporal variations in the life-curve with seasonal and with climatic periodicities.
The Arctic tundra is marked by a long dark winter of bitter cold and a short nightless summer of intense illumination, there fore it is reasonable to postulate a prolonged process of elimination as the climate changed—an elimination of those types which did not vary in the direction of quick-flowering and quick-fruiting, dying down in winter, dispensing with all but a little water, and accumulating stores in underground parts.
Among the features of the life-curve that may be tentatively associated with climatic changes, the following may be suggested —the length of the mammalian gestation and the season for giving birth; the periodicities of migrating birds; the interpola tion of periods of winter sleep, rest, coma, lethargy and even de differentiation. Every trajectory of life should be looked at in the light of the evolution of climates.
Often in the history of the earth a change towards great cold has involved severe elimination. In humid periods there tends to be abundant succulent fodder for browsing animals and exten sive forests affording shelter. Diminution of moisture, if it does not go too far, favours the increase of grasses and of grazing animals. Aridity makes the forests shrink and prompts the search for new haunts. The times of quickening, the "expression-points" or "pulsations" of evolution, may often be correlated with climatic changes, chiefly in temperature and humidity, due sometimes to topographic, at others to general atmospheric, conditions. Behind these, again, lie larger factors still, such as shrinkages of the earth's crust.
Interaction of Endemic and Introduced Faunas.—(i) The newcomers may destroy the indigenous or previously natu ralized forms. Thus the mongoose, introduced into Jamaica, de stroyed the indigenous "cane-rats" and the alien ship-rats. The introduced animals may become so numerous that they make life difficult for their predecessors though they do not actually devour them. They may, for instance, seriously reduce the f ood supply, but they may be prejudicial with varying degrees of directness. When the brown rat (Rattus norvegicus) found its way to Britain in the early i8th century, it proved itself hardier, more plastic, and more fecund than the black rat (Rattus rattus), with the result that in 5o years the latter was almost exterminated except in places where it was continually being re-introduced by ships.
• Sometimes the influence is more subtle. Thus an extension of squirrels into an area may be followed by the reduction of the number of wood-pigeons. There does not seem at first sight any intersection of the two lives; but squirrels, though largely vege tarian, cannot resist killing and eating the young squabs in the nest; and this is, from the farmer's point of view, a useful check.
The introduction of an animal into a new country may involve the introduction of its parasites; thus rats harbour rat-fleas which disseminate bubonic plague, and it is from rats that pigs, and thence men, become infected with the disease of trichinosis, which is due to Trichinella spiralis, a small Nematode. Many of these inter-relations are very subtle; thus the problem of getting rid of Bilharziasis in Durban is easier than it is in Japan, for in South Africa the only host of the adult parasite is man, whereas in Japan it also occurs in cattle. In both countries the juvenile stages are spent in various water-snails, and their abundance or rarity in turn is correlated with the presence of water-birds which feed upon them and of water-plants on which they feed.
Apart from two species of bats, it is doubtful if there are any indigenous mammals in New Zealand; but 48 species have been introduced, 44 purposely and four accidentally. The four com prise the mouse and three rats, one of which, the Maori rat (Mus exulans), has disappeared since European settlement began. Twenty-five of the 48 species of mammals are at present well established and feral in certain districts—wallaby, common opos sum, sooty opossum, pig, horse, various deer, cattle, sheep, goats, chamois, cat, ferret, stoat, weasel, black rat, brown rat, mouse, rabbit, hare and hedgehog.
About 13o species of birds have been purposely introduced into New Zealand since the date of Captain Cook's landing, and have become truly wild, such as mallard, pheasant, pigeon, skylark, thrush, blackbird, hedge sparrow, rook, starling, Indian mynah, house sparrow, chaffinch, goldfinch, greenfinch and yellow hammer.
On the other hand, since 1868, nine species of birds have become either very rare or extinct, such as native crows, huia, native thrushes, the burrowing parrot Stringops, the native quail, and the white heron. Others, once abundant, have been driven back into areas where there has not been much settlement. As to the causes, Thomson writes : "It must not be supposed that it is the introduced animals alone which have produced this effect, even though rats, cats, rabbits, pigs, cattle, stoats and weasels, as well perhaps as some kinds of introduced birds, have penetrated beyond the settled districts. It is largely the direct disturbance of their haunts and breeding-places, and the interference with their food supply, which has caused this destruction and diminution of the native fauna." What is true for birds holds also for lower animals, from lizards to insects; but again the reasons are to be found in human inter vention rather than in direct competition with newcomers. This is corroborated by the fact that there have been some notable cases of increase during the last 5o years. Thus the bell-bird has become abundant in the South island, though scarce in the North; and the harrier has greatly increased, perhaps in relation to the abundance of young rabbits. The wax-eye or blight bird (Zosterops coeruleus) has apparently increased greatly since first recorded in 1832, perhaps in relation to the supply of animal food about houses and stockyards. The case of the long-tailed cuckoo (Uro dynamis taitensis) is interesting as an illustration of the complex ity of inter-relations. It seems to have become increasingly numerous during the past 3o years, and this is attributed to the increase of small European birds, whose eggs and young it eats, and also to the food afforded in and about trout-hatcheries.
Except for some indications, not yet fully worked out, in Inch nadamph caves in Sutherlandshire, there is no evidence of Palaeo lithic man in Scotland, though he lived in Britain farther south; the first-comers to Scotland, perhaps 8,000 years ago, were Neo lithic. What higher animals greeted Neolithic man when he arrived in Scotland? The answer is : the present mammalian fauna, minus some that have been introduced later, such as rab bits, rats, and domestic mammals, and plus some that have been lost, such as reindeer, bear, and wolf. As regards their origin, the original Scottish mammals, present when man established himself, might be grouped in three contingents : (a) those distinctively arctic, like the reindeer and the lemming; (b) those of the forests, like the red deer and the elk; and (c) those of the plains like the hare and the wild horse.
Reindeer, probably of the woodland or caribou variety, persisted as wild animals in the north of Scotland until the a 2th century. Beside the reindeer may be included the giant fallow deer (Irish elk) and the true elk, the former extinct, the latter no longer represented in Britain. The red deer and the roe deer are per sistent representatives of the old fauna; the fallow deer is a subsequent introduction.
Appearing first in inter-glacial deposits, but lasting for many centuries along with man, was the wild ox or urus (Bos primige nius), once widespread in Europe. Man did not domesticate it in Scotland, but it may have crossed with the Celtic shorthorn (Bos taurus longifrons) which he brought with him when he came. The wild boar (Sus scrofa), on the other hand, was one of the early Scottish mammals, doubtless spreading from the forests of central Europe, and it was domesticated in Scotland as elsewhere. There were wild horses in Scotland before man arrived—the Celtic pony type (Equus agilis), common long ago in western Europe ; and traces of these linger in the Hebridean pony and the Shetland pony. But the main stock of domesticated horses prob ably came in the train of the Neolithic herdsmen from the con tinent, notably perhaps from Scandinavia.
As to the larger carnivores, the wolf lingered in Scotland till the 17th century, even to 1743, according to some. Its disappearance was especially connected with the shrinkage of the forests, and with more determined wolf-hunting, which was compulsory as late as the 15th century. The brown bear lingered in Scotland into the Christian era, and with it the lynx.
Of the original small carnivores there is a persistence of fox, badger, otter, polecat, stoat, weasel, wild cat and marten. The last is dwindling to the vanishing point; the wild cat is rare, but of recent years notably on the increase. Seals continue to hold their own, but the walrus, which used often to visit Scottish coasts, has been persecuted into extreme rarity. Most cetaceans, except porpoises, become scarcer every year.
Among rodents, the beaver and the lemming have disappeared; the common hare and the mountain hare have increased, partly through, preservation and partly because of greater abundance of food afforded by the spread of agriculture. The rabbit is an instance of naturalization, for it was unknown in Britain before the Norman conquest. Both the black rat and the brown rat are aliens, the former dating from the time of the Crusaders and the second from about the middle of the 18th century. The small native rodents, the voles and mice, hold their own, and so do the insectivores—the hedgehog, the mole and the shrews.
Taking a general survey, we see that there has been in Scotland an introduction of domesticated sheep, cattle, horses, fallow deer, dog, cat, poultry, pheasants and a few more The wild rabbits were probably introduced deliberately, the rats unwittingly. Since man settled in Scotland the fauna has lost about 14 species of mammals and birds, and while Ritchie points out that the total number of species is actually greater, the standard is lower. The noble has been exchanged for the ignoble—the elk for the earth worm; the visible has been exchanged for the invisible—the beaver for the bug! The case of Scotland is instructive since the area is small and the fauna well-known, and Dr. Ritchie's careful study shows the complexity of the factors that are often involved in faunistic change. Besides the alterations in climate, there are those involved in deforestation, drainage, the extension of arable land, and the reduction of wild corners. Much has depended on man's attitude to the wild life of the country, for he sometimes preserves and sometimes exterminates. The introduction of new types has, no doubt, its influence on those in possession, but this is often far from being a case of sheer competition.
Where man plays a large part two main processes may be re cognized. On the one hand, there is a diminution in the number of larger, more highly evolved types, partly because they are apt to be dangerous to human life or destructive of stock and crops, partly because of fashion and a pride in trophies, but partly as the result of slow processes such as deforestation and the spread of agriculture. On the other hand, "while man lops off giants at the head of the scale he adds insignificant pigmies at the bottom," partly by carelessness, as in the case of rats and cockroaches, partly by thoughtlessness, as in the case of rabbits in Australia or sparrows in the United States.
Willis notes that man has often failed in naturalization by attempting too much abruptly. Learning from failure, he is now trying gradual transitions, "as in the way he has treated Liberian coffee in Java, taking the seed of successive generations a few score yards higher up each time, till he has persuaded the tree to do well at a much higher elevation than that to which it is naturally suited." The attempts to acclimatize the beautiful Cyperus papyrus ,in the Ceylon Botanic Garden failed when seed from Europe was used, but seed from Saharanpur in India succeeded at once. The moral is that man must moderate his impatience and take a hint at least from Nature's operations by small steps throughout long periods.
(J. A. TH.)