Family31.—TORNATELLIDÆ. (7 Genera, 62 Species living, 166 fossil.)
Family32.—BULLIDÆ. (12 Genera, 168 Species living, 88 fossil.)
Family33.—APHYSIADÆ. (8 Genera, 84 Species living, 4 fossil.)
Family34.—PLEUROBRANCHIDÆ. (7 Genera, 28 Species living, 5 fossil.)
Family35.—PHYLLIDIADÆ. (4 Genera, 14 Species living, 0 fossil.)
Family36.—DORIDÆ. (23 Genera, 160 Species living, 0 fossil.)
Family37.—TRITONIADÆ. (9 Genera, 38 Species living, 0 fossil.)
Family38.—ÆOLIDÆ. (14 Genera, 101 Species living, 0 fossil.)
Family39.—PHYLLYRHOIDÆ. (1 Genus, 6 Species living, 0 fossil.)
Family40.—ELYSIADÆ. (5 Genera, 13 Species living, 0 fossil.)
Order IV.—NUCLEO-BRANCHIATA.
These are oceanic, swimming molluscs, of a delicate texture. They are found in all warm seas, and range back to the Lower Silurian epoch. There are only two families.
Family41.—FIROLIDÆ. (2 Genera, 33 Species living, 1 fossil.)
Family42.—ATLANTIDÆ. (5 Genera, 22 Species living, 159 fossil.)
Class.—PTEROPODA.
These are swimming, oceanic mollusca, inhabiting both Arctic, Temperate, and Tropical seas. The three families have each a wide distribution in all the great oceans. They range back to the Silurian period.
Family1.—HYALEIDÆ. (9 Genera, 52 Species living, 95 fossil.)
Family2.—LIMACINIDÆ. (4 Genera, 19 Species living, 0 fossil.)
Family3.—CLIONIDÆ. (4 Genera, 14 Species living, 0 fossil.)
Class.—BRACHIOPODA.
These are sedentary, bivalve, marine mollusca, having laterally symmetrical shells, but with unequal valves. Both in space and time they are the most widely distributed molluscs. They are found in all seas, and at all depths; and when any of the families or genera have a restricted range, it seems to be due to our imperfect knowledge, rather than to any real geographical limitations. In time they range back to the Cambrian formation, and seem to have had their maximum development in the Silurian period. It is not, therefore, necessary for our purpose, to do more than give the names of the families with the numbers of the genera and species, as before.
Family1.—TEREBRATULIDÆ. (5 Genera, 67 Species living, 340 fossil.)
Family2.—SPIRIFERIDÆ. (4 Genera, 0 Species living, 380 fossil.)
Family3.—RHYNCHONELLIDÆ. (3 Genera, 4 Species living, 422 fossil.)
Family4.—ORTHIDÆ. (4 Genera, 0 Species living, 328 fossil.)
Family5.—PRODUCTIDÆ. (3 Genera, 0 Species living, 146 fossil.)
Family6.—CRANIADÆ. (1 Genus, 5 Species living, 37 fossil.)
Family7.—DISCINIDÆ. (2 Genera, 10 Species living, 90 fossil.)
Family8.—LINGULIDÆ. (2 Genera, 16 Species living, 99 fossil.)
Class.—CONCHIFERA.
The Conchifera, or ordinary Bivalve Molluscs, may be distinguished from the Brachiopoda by having their shells laterally unsymmetrical, while the valves are generally (but not always) equal. They are mostly marine, but a few inhabit fresh water. As the distribution of some of the families presents points of interest, we shall treat them in the same manner as the marine Gasteropoda.
Family1.—OSTREIDÆ. (5 Genera, 426 Species.)
Distribution.—The Ostreidæ, including the Oysters and Scallops, are found in all seas, Arctic as well as Tropical. There are nearly 1,400 species fossil, ranging back to the Carboniferous period.
Family2.—AVICULIDÆ. (3 Genera, 94 Species.)
Distribution.—The Aviculidæ, or Wing-shells and Pearl Oysters, are characteristic of Tropical and warm seas, a few only ranging into temperate regions. Nearly 700 fossil species are known from various formations ranging back to the Devonian, and Lower Silurian.
Family3.—MYTILIDÆ. (3 Genera, 217 Species.)
Distribution.—The Mytilidæ, or Mussels, have a world-wide distribution. There is one fresh-water species, which inhabits the Volga. There are about 350 fossil species, ranging back to the Carboniferous epoch.
Family4.—ARCADÆ. (6 Genera, 360 Species.)
Distribution.—The Arcadæ are universally distributed, and are most abundant in warm seas. The genusLedais, however, abundant in Arctic and Temperate regions, andSolenellais confined to the South Temperate zone. There are near 1,200 fossil species, found in all strata as low as the Lower Silurian.
Family5.—TRIGONIADÆ. (1 Genus, 3 Species.)
Distribution.—The livingTrigoniæare confined to Australia, but there are 5 other genera fossil, containing about 150 species, and found in various formations from the Chalk to the Lower Silurian.
Family6.—UNIONIDÆ. (7 Genera, 549 Species.)
Distribution.—The Unionidæ, or Fresh-water Mussels, are found in all the fresh waters of the globe, but some of the genera are restricted.Castalia,MycetopusandMulleriaare confined to the rivers of South America;Anodon, to the Nearctic and Palæarctic regions;Iridina, andEtheria, to the rivers of Africa;Uniohas a universal distribution, but is especially abundant in North America. About 60 fossil species are found in the Tertiary and Wealden formations.
Family7.—CHAMIDÆ. (1 Genus, 50 Species.)
Distribution.—The Chamidæ, or Giant Clams, are confined to Tropical seas, chiefly among coral reefs. There are two other genera and 62 species fossil, ranging from the Chalk to the Oolite formations.
Family8.—HIPPURITIDÆ. (5 Genera, 103 Species.)
Fossils of doubtful affinity, from the Chalk formation.
Family9.—TRIDACNIDÆ. (1 Genus, 8 Species.)
Distribution.—The Tridacnidæ, or Clam-shells, are of very large size, and are confined to the Tropical regions of the Indian and Pacific Oceans. A few species have been found fossil in the Miocene formation.
Family10.—CARDIADÆ. (1 Genus, 200 Species.)
Distribution.—The Cardiadæ, or Cockles, are of world-wide distribution. Another genus is fossil, and nearly 400 fossil species are known, ranging back to the Upper Silurian formation.
Family11.—LUCINIDÆ (8 Genera, 178 Species.)
Distribution.—The Lucinidæ inhabit the Tropical and Temperate seas of all parts of the world; but the genusCorbisis confined to the Indian and Pacific Oceans,MontacutaandLepton, to the Atlantic. There are nearly 500 extinct species, ranging from the Tertiary back to the Silurian formation.
Family12.—CYCLADIDÆ. (3 Genera, 176 Species.)
Distribution.—The Cycladidæ are small fresh- or brackish-water shells found all over the globe. The genusCyclasis most abundant in the North Temperate zone, whileCyrenainhabits the warmer shores of the Atlantic and Pacific, but is absent from the West Coast of America. There are about 150 species fossil, ranging back from the Pliocene to the Wealden formations.
Family13.—CYPRINIDÆ. (10 Genera, 176 Species).
Distribution.—Universal.CyprinaandAstarteare Arctic and North Temperate;Carditais Tropical and South Temperate. There are several extinct genera and about 1,000 species found in all formations as far back as the Lower Silurian.
Family14.—VENERIDÆ. (10 Genera, 600 Species.)
Distribution.—Universal.Lucinopsisis confined to the North Atlantic;Glauconezato the months of rivers in the Oriental region;MeroeandTrigonato warm seas. There are about 350 fossil species, ranging back to the Oolitic period.
Family15.—MACTRIDÆ. (5 Genera, 147 Species.)
Distribution.—All seas, but more abundant in the Tropics.Gnathodonis found in the Gulf of Mexico;Anatinellain the Oriental region. There are about 60 fossil species, ranging back to the Carboniferous period.
Family16.—TELLINIDÆ. (11 Genera, 560 Species.)
Distribution.—All seas; most abundant in the Tropics.Galateais confined to African rivers. There are about 60 fossil species, mostly Tertiary, but ranging back to the Carboniferous period.
Family17.—SOLENIDÆ. (3 Genera, 63 Species.)
Distribution.—All Temperate and Tropical seas. There are 80 fossil species which range back to the Carboniferous epoch.
Family18.—MYACIDÆ. (6 Genera, 121 Species.)
Distribution.—All seas.Panopæainhabits both North and South Temperate seas;Glycimeris, Arctic seas. There are near 350 fossil species, ranging back to the Lower Oolite formation.
Family19.—ANATINIDÆ. (8 Genera, 246 Species.)
Distribution.—All seas.Pholadomyais from Tropical Africa;Myadorafrom the Western Pacific;MyochamaandChamostræaare Australian. There are about 400 fossil species, ranging back to the Lower Silurian formation.
Family20.—GASTROCHÆNIDÆ. (5 Genera, 40 Species.)
Distribution.—Temperate and warm seas.Aspergillumranges from the Red Sea to New Zealand. There are 35 fossil species, ranging back to the Lower Oolite.
Family21.—PHOLADIDÆ (4 Genera, 81 Species.)
Distribution.—These burrowing molluscs inhabit all Temperate and warm seas from Norway to New Zealand. There are about 50 fossil species, ranging back to the epoch of the Lias.
General Remarks on the Distribution of the Marine Mollusca.
The marine Mollusca are remarkable for their usually wide distribution. About 48 of the families are cosmopolitan, ranging over both hemispheres, and in cold as well as warm seas. About 15 are restricted to the warmer seas of the globe; but several of these extend from Norway to New Zealand, a distribution which may be called universal, and only 2 or 3 are absolutely confined to Tropical seas. Two small families only, are confined to the Pacific and Indian Oceans. Marine fishes, on the other hand, have a much less cosmopolitan character, no less than 30 families having a limited distribution, while 50 are universal. Some of these 30 families are confined to the Northern seas, some to the Atlantic and Mediterranean, and a considerable number to the Indian Ocean and Western Pacific. Many of these families, it is true, are much smaller than those of the Mollusca, which seem to possess very few of those small isolated families of two or three species only, which abound in all the Vertebrate classes. These differences are no doubt connected with the higher organisation of fishes, which renders them more susceptible to changed conditions of life; and this is indicated by the much less antiquity of existing families of fishes, the greater part of which do not date back beyond the Cretaceous epoch, and many of them only to the Eocene. In striking contrast we have the vast antiquity of most of the families ofMollusca, as shown in the following table of their range taken from Mr. Woodward's work, but re-arranged, and somewhat modified.
Nor is this enormous antiquity confined to family types alone. Many genera are equally ancient. The genusLingulahasexisted from the earliest Palæozoic times down to the present day; whileTerebratula,Rhynchonella,Discina,Nautilus,Natica,Pleurotomaria,Patella,Dentalium,Mytilusand many other living forms, range back to the Palæozoic epoch. That groups of such immense antiquity, and having power to resist such vast changes of external conditions as they must have been subject to, should now be widely distributed, is no more than might reasonably be expected. It is only in the case of sub-genera and species, that we can expect the influence of recent geological or climatal changes to be manifest; and it must be left to special students to work out the details of their distribution, with reference to the general principles found to obtain among the more highly organised animals.
SUMMARY OF THE DISTRIBUTION, AND LINES OF MIGRATION, OF THE SEVERAL CLASSES OF ANIMALS.
Having already given summaries of the distribution of the several orders, and of some of the classes of land animals, we propose here to make a few general remarks on the special phenomena presented by the more important groups, and to indicate where possible, the general lines of migration by which they have become dispersed over wide areas.
Mammalia.
This class is very important, and its past history is much better known than that of most others. We shall therefore briefly summarise the results we have arrived at from our examination of the distribution of extinct and living forms of each order.
Primates.—This order, being pre-eminently a tropical one, became separated into two portions, inhabiting the Eastern and Western Hemispheres respectively, at a very early epoch. In consequence of this separation it has diverged more radically than most other orders, so that the two American families, Cebidæ and Hapalidæ, are widely differentiated from the Apes, Monkeys, and Lemurs of the Old World. The Lemurs were probably still more ancient, but being much lower in organisation, they became extinct in most of the areas where the higher forms of Primates became developed. Remains found in the Eocene formation indicate, that the North American and EuropeanPrimates had, even at that early epoch, diverged into distinct series, so that we must probably look back to the secondary period for the ancestral form from which the entire order was developed.
Chiroptera.—These are also undoubtedly very ancient. The most generalised forms—the Vespertilionidæ and Noctilionidæ—are the most widely distributed; while special types have arisen in America, and in the Eastern Hemisphere. Remains found in the Upper Eocene formation of Europe differ little from species still living in the same countries; so that we can form no conjecture as to the origin or migration, of the group. Their power of flight would, however, enable them rapidly to spread over all the great continents of the globe.
Insectivora.—This very ancient group, now probably verging towards extinction, appears to have originated in the Northern continent, and never to have reached Australia or South America. It may, however, have become extinct in the latter country owing to the competition of the numerous Edentata. The Insectivora now often maintain themselves amidst more highly developed forms, by means of some special protection. Some burrow in the earth,—like the moles; others have a spiny covering,—as the hedgehog's and several of the Centetidæ; others are aquatic,—-as thePotamogaleand the desman; others have a nauseous odour,—as the shrews; while there are several which seem to be preserved by their resemblance to higher forms,—as the elephant-shrews to jerboas, and the tupaias to squirrels. The same need of protection is shown by the numerous Insectivora inhabiting Madagascar, where the competing forms are few; and by one lingering in the Antilles, where there are hardly any other mammalia.
Carnivora.—Although perhaps less ancient than the preceding, this form of mammal is far more highly organised, and from its earliest appearance appears to have become dominant in the world. It would therefore soon spread widely, and diverge into the various specialised types represented by existing families. Most of these appear to have originated in the Eastern Hemisphere, the only Carnivora occurring in NorthAmerican Miocene deposits being ancestral forms of Canidæ and Felidæ. It seems probable, therefore, that the order had attained a considerable development before it reached the Western Hemisphere. The Procyonidæ, now confined to America, are not very ancient; and the occurrence of a few allied forms in the Himalayas (ÆlurusandÆluropus) render it probable that their common ancestors entered North America from the Palæarctic region during the Miocene period, but being a rather low type they have succumbed under the competition of higher forms in most parts of the Eastern Hemisphere. Bears and Weasels are probably still more recent emigrants to America. The aquatic carnivora (Seals, &c.) are, as might be expected, more widely and uniformly distributed, but there is little evidence to show at what period the type was first developed.
Ungulata.—These are the dominant vegetable-feeders of the great continents, and they have steadily increased in numbers and in specialisation from the oldest Tertiary times to the present day. Being generally of larger size and less active than the Carnivora, they have somewhat more restricted powers of dispersal. We have good evidence that their wide range over the globe is a comparatively recent phenomenon. Tapirs and Llamas have probably not long inhabited South America, while Rhinoceroses and Antelopes were once, perhaps, unknown in Africa, although abounding in Europe and Asia. Swine are one of the most ancient types in both hemispheres; and their great hardiness, their omnivorous diet, and their powers of swimming, have led to their wide distribution. The sheep and goats, on the other hand, are perhaps the most recent development of the Ungulata, and they seem to have arisen in the Palæarctic region at a time when its climate already approximated to that which now prevails. Hence they are pre-eminently a Temperate group, never found within the Tropics except upon a few mountain ranges.
Proboscidea.—These huge animals (the Elephants and Mastodons) appear to have originated in the warmer parts of the Palæarctic region, but they soon spread over all the greatcontinents, even reaching the southern extremity of America. Their extinction has probably depended more on physical than on organic changes, and we can clearly trace their almost total disappearance to the effects of the Glacial epoch.
Rodentia.—Rodents are a very dominant group, and a very ancient one. Owing to their small size and rapid powers of increase, they soon spread over almost every part of the globe, whence has resulted a great specialisation of family types in the South American continent which remained so long isolated. They are capable of living wherever there is any kind of vegetable food, hence their range will be determined rather by organic than by physical conditions; and the occupation of a country by enemies or by competing forms, is probably the chief cause which has prevented many of the families from acquiring a wide range. The occurrence of isolated species of the South American families, Octodontidæ and Echimyidæ in the Ethiopian and Palæarctic regions, is an indication that the range of many of the families has recently become less extensive.
Edentata.—These singular and lowly-organised animals appear to have become almost restricted to the two great Southern lands—South Africa and South America—at an early period; and, being there free from the competition of higher forms, developed a number of remarkable types often of huge size, of which the Megatherium is one of the best known. The incursion of the highly-organised Ungulates and Carnivora into Africa during the Miocene epoch, probably exterminated most of them in that continent; but in America they continued in full force down to the Post-Pliocene period; and even now, the comparatively diminutive Sloths, Ant-eaters, and Armadillos, form a large and important portion of the fauna.
Marsupialia and Monotremata.—These are probably the representatives of the most ancient and lowly-organised types of mammal. They once existed in the northern continents, whence they spread into Australia; and being isolated, and preserved from the competition of the higher forms which soon arose in other parts of the world, they have developed into a variety of types, which, however, still preserve a generaluniformity of organisation. One family, which continued to exist in Europe till the latter part of the Miocene period, reached America, and has there been preserved to our day.
Lines of Migration of the Mammalia.—The whole series of phenomena presented by the distribution of the Mammalia, looked at broadly, are in harmony with the view that the great continents and oceans of our own epoch have been in existence, with comparatively small changes, during all Tertiary times. Each one of them has, no doubt, undergone considerable modifications in its area, its altitude, and in its connection with other lands. Yet some considerable portion of each continent has, probably, long existed in its present position, while the great oceans seem to have occupied the same depressions of the earth's crust (varied, perhaps, by local elevations and subsidences) during all this vast period of time. Hence, allowing for the changes of which we have more or less satisfactory evidence, the migrations of the chief mammalian types can be pretty clearly traced. Some, owing to their small size and great vitality, have spread to almost all the chief land masses; but the majority of the orders have a more restricted range. All the evidence at our command points to the Northern Hemisphere as the birth-place of the class, and probably of all the orders. At a very early period the land communication with Australia was cut off, and has never been renewed; so that we have here preserved for us a sample of one or more of the most ancient forms of mammal. Somewhat later the union with South America and South Africa was severed; and in both these countries we have samples of a somewhat more advanced stage of mammalian development. Later still, the union by a northern route between the Eastern and Western Hemispheres appears to have been broken, partly by a physical separation, but almost as effectually by a lowering of temperature. About the same period the separation of the Palæarctic region from the Oriental was effected, by the rise of the Himalayas and the increasing contrast of climate; while the formation of the great desert-belts of the Sahara, Arabia, Persia, and Central Asia, helped to complete the separation ofthe Temperate and Tropical zones, and to render further intermigration almost impossible.
In a few cases—of which the Rodents in Australia and the pigs in Austro-Malaya are perhaps the most striking examples—the distribution of land-mammals has been effected by a sea-passage either by swimming or on floating vegetation; but, as a rule, we may be sure that the migrations of mammalia have taken place over the land; and their presence on islands is, therefore, a clear indication that these have been once connected with a continent. The present class of animals thus affords the best evidence of the past history of the land surface of our globe; and we have chiefly relied upon it in sketching out (in Part III.) the probable changes which each of our great regions has undergone.
Birds.
Although birds are, of all land-vertebrates, the best able to cross seas and oceans, it is remarkable how closely the main features of their distribution correspond with those of the Mammalia. South America possesses the low Formicaroid type of Passeres,—which, compared with the more highly developed forms of the Eastern Hemisphere, is analogous to the Cebidæ and Hapalidæ as compared with the Old World Apes and Monkeys; while its Cracidæ as compared with the Pheasants and Grouse, may be considered parallel to the Edentata as compared with the Ungulates of the Old World. The Marsupials of America and Australia, are paralleled among birds, in the Struthionidæ and Megapodiidæ; the Lemurs and Insectivora preserved in Madagascar are represented by the Mascarene Dididæ; the absence of Deer and Bears from Africa is analogous to the absence of Wrens, Creepers, and Pheasants; while the African Hyracidæ and Chrysochloridæ among mammals, may well be compared with the equally peculiar Coliidæ and Musophagidæ among birds.
From these and many other similarities of distribution, it is clear that birds have, as a rule, followed the same great lines of migration as mammalia; and that oceans, seas, and deserts, havealways to a great extent limited their range. Yet these barriers have not been absolute; and in the course of ages birds have been able to reach almost every habitable land upon the globe. Hence have arisen some of the most curious and interesting phenomena of distribution; and many islands, which are entirely destitute of mammalia, or possess a very few species, abound in birds, often of peculiar types and remarkable for some unusual character or habit. Striking examples of such interesting bird-faunas are those of New Zealand, the Sandwich Islands, the Galapagos, the Mascarene Islands, the Moluccas, and the Antilles; while even small and remote islets,—such as Juan Fernandez and Norfolk Island, have more light thrown upon their past history by means of their birds, than by any other portion of their scanty fauna.
Another peculiar feature in the distribution of this class is the extraordinary manner in which certain groups and certain external characteristics, have become developed in islands, where the smaller and less powerful birds have been protected from the incursions of mammalian enemies, and where rapacious birds—which seem to some degree dependent on the abundance of mammalia—are also scarce. Thus, we have the Pigeons and the Parrots most wonderfully developed in the Australian region, which is pre-eminently insular; and both these groups here acquire conspicuous colours very unusual, or altogether absent, elsewhere. Similar colours (black and red) appear, in the same two groups, in the distant Mascarene islands; while in the Antilles the parrots have often white heads, a character not found in the allied species on the South American continent. Crests, too, are largely developed, in both these groups, in the Australian region only; and a crested parrot formerly lived in Mauritius,—a coincidence too much like that of the colours as above noted, to be considered accidental.
Again, birds exhibit to us a remarkable contrast as regards the oceanic islands of tropical and temperate latitudes; for while most of the former present hardly any cases of specific identity with the birds of adjacent continents, the latter often show hardly any differences. The Galapagos and Madagascarare examples of the first-named peculiarity; the Azores and the Bermudas of the last; and the difference can be clearly traced to the frequency and violence of storms in the one case and to the calms or steady breezes in the other.
It appears then, that although birds do not afford us the same convincing proof of the former union of now disjoined lands as we obtain from mammals, yet they give us much curious and suggestive information as to the various and complex modes in which the existing peculiarities of the distribution of animals have been brought about. They also throw much light on the relation between distribution and the external characters of animals; and, as they are often found where mammalia are quite absent, we must rank them as of equal value for the purposes of our present study.
Reptiles.
These hold a somewhat intermediate place, as regards their distribution, between mammals and birds, having on the whole rather a wider range than the former, and a more restricted one than the latter.
Snakes appear to have hardly more facilities for crossing the ocean than mammals; hence they are generally absent from oceanic islands. They are more especially a tropical group, and have thus never been able to pass from one continent to another by those high northern and southern routes, which we have seen reason to believe were very effectual in the case of mammalia and some other animals. Hence we find no resemblance between the Australian and Neotropical regions, or between the Palæarctic and Nearctic; while the Western Hemisphere is comparatively poor as regards variety of types, although rich in genera and species. Deserts and high mountains are also very effectual barriers for this group, and their lines of migration have probably been along river valleys, and occasionally across narrow seas by means of floating vegetation.
Lizards, being somewhat less tropical than snakes, may have passed by the northern route during warm epochs. They are also more suited to traverse deserts, and they possess some unknownmeans of crossing the ocean, as they are not unfrequently found in remote oceanic islands. These various causes have modified their distribution. The Western Hemisphere is much richer in lizards than it is in snakes; and it is also very distinct from the Eastern Hemisphere. The lines of migration of lizards appear to have been along the mountains and deserts of tropical countries, and, under special conditions, across tropical seas from island to island.
Crocodiles are a declining group. They were once more generally distributed, all the three families being found in British Eocene deposits. Being aquatic and capable of living in the sea, they can readily pass along all the coasts and islands of the warmer parts of the globe. Tortoises are equally ancient, and the restriction of certain groups to definite areas seems to be also a recent phenomenon.
Amphibia.
The Amphibia differ widely from Reptiles in their power of enduring cold; one of their chief divisions, the Urodela or Tailed-Batrachia, being confined to the temperate parts of the Northern Hemisphere. To this class of animals the northern and southern routes of migration were open; and we accordingly find a considerable amount of resemblance between South America and Australia, and a still stronger affinity between North America and the Palæarctic continent. The other tropical regions are more distinct from each other; clearly indicating that, in this group, it is tropical deserts and tropical oceans which are the barriers to migration. The class however is very fragmentary, and probably very ancient; so that descendants of once widespread types are now found isolated in various parts of the globe, between which we may feel sure there has been no direct transmission of Batrachia. Remembering that their chief lines of migration have been by northern and southern land-routes, by floating ice, by fresh-water channels, and perhaps at rare intervals by ova being carried by aquatic birds or by violent storms,—we shall be able to comprehend most of the features of their actual distribution.