It is assumed by some writers that the breeding-place of a species is to be considered as its true home rather than that to which it retires in winter; but this can hardly be accepted as a rule of universal application. A bird can only breed successfully where it can find sufficient food for its young; and the reason probably why so many of the smaller birds leave the warm southern regions to breed in temperate or even cold latitudes, is because caterpillars and other soft insect larvæ are there abundant at the proper time, while in their winter home the larvæ have all changed into winged insects. But this favourable breeding district will change its position with change of climate; and as the last great change has been one of increased warmth in all the temperate zones, it is probable that many of the migratory birds are comparatively recent visitors. Other changes may however have taken place, affecting the vegetation and consequently the insects of a district; and we have seldom the means of determining in any particular case in what direction the last extension of range occurred. For the purposes of the study of geographical distribution therefore, we must, except in special cases, consider the true range of a species to comprise all the area which it occupies regularly for any part of the year, while all those districts which it only visits at more or less distant intervals, apparently driven by storms or by hunger, and where it never regularly or permanently settles, should not be included as forming part of its area of distribution.
Means of Dispersal of Reptiles and Amphibia.—If we leave out of consideration the true marine groups—the turtles and sea-snakes—reptiles are scarcely more fitted for traversing seas and oceans than are mammalia. We accordingly find that in those oceanic islands which possess no indigenous mammals, land reptiles are also generally wanting. The several groups of these animals, however, differ considerably both in their means of dispersal and in their power of resisting adverse conditions. Snakes are most dependent on climate, becoming very scarce in temperate and cold climates and entirely ceasing at 62° north latitude, and they do not ascend very lofty mountains, ceasing at 6,000 feet elevation in the Alps. Some inhabit deserts, others swamps and marshes, while many are adapted for a life in forests. They swim rivers easily, but apparently have no means of passing the sea, since they are very rarely found on oceanic islands. Lizards are also essentially tropical, but they go somewhat farther north than snakes, and ascend higher on the mountains, reaching 10,000 feet in the Alps. They possess too some unknown means (probably in the egg-state) of passing over the ocean, since they are found to inhabit many islands where there are neither mammalia nor snakes.
The amphibia are much less sensitive to cold than are true reptiles, and they accordingly extend much farther north, frogs being found within the arctic circle. Their semi-aquatic life also gives them facilities for dispersal, and their eggs are no doubt sometimes carried by aquatic birds from one pond or stream to another. Salt water is fatal to them as well as to their eggs, and hence it arises that they are seldom found in those oceanic islands from which mammalia are absent. Deserts and oceans would probably form the most effectual barriers to their dispersal; whereas both snakes and lizards abound in deserts, and have some means of occasionally passing the ocean which frogs and salamanders do not seem to possess.
Means of Dispersal of Fishes.—The fact that the same species of freshwater fish often inhabit distinct river systems, proves that they have some means of dispersal over land. The many authentic accounts of fish falling from the atmosphere, indicate one of the means by which they may be transferred from one river basin to another, viz., by hurricanes and whirlwinds, which often carry up considerable quantities of water and with it fishes of small size. In volcanic countries, also, the fishes of subterranean streams may sometimes be thrown up by volcanic explosions, as Humboldt relates happened in South America. Another mode by which fishes may be distributed is by their eggs being occasionally carried away by aquatic birds; and it is stated by Gmelin that geese and ducks during their migrations feed on the eggs of fish, and that some of these pass through their bodies with their vitality unimpaired.[[2]] Even water-beetles flying from one pond to another might occasionally carry with them some of the smaller eggs of fishes. But it is probable that fresh-water fish are also enabled to migrate by changes of level causing streams to alter their course and carry their waters into adjacent basins. On plateaux the sources of distinct river systems often approach each other, and the same thing occurs with lateral tributaries on the lowlands near their mouths. Such changes, although small in extent, and occurring only at long intervals, would act very powerfully in modifying the distribution of fresh-water fish.
Sea fish would seem at first sight to have almost unlimited means of dispersal, but this is far from being the case. Temperature forms a complete barrier to a large number of species, cold water being essential to many, while others can only dwell in the warmth of the tropics. Deep water is another barrier to large numbers of species which are adapted to shores and shallows; and thus the Atlantic is quite as impassable a gulf to most fishes as it is to birds. Many sea fishes migrate to a limited extent for the purpose of depositing their spawn in favourable situations. The herring, an inhabitant of the deep sea, comes in shoals to our coast in the breeding season; while the salmon quits the northern seas and enters our rivers, mounting upwards to the clear cold water near their sources to deposit its eggs. Keeping in mind the essential fact that changes of temperature and of depth are the main barriers to the dispersal of fish, we shall find little difficulty in tracing the causes that have determined their distribution.
Means of Dispersal of Mollusca.—The marine, fresh-water, and land mollusca are three groups whose powers of dispersal and consequent distribution are very different, and must be separately considered. The Pteropoda, the Ianthina, and other groups of floating molluscs, drift about in mid-ocean, and their dispersal is probably limited chiefly by temperature, but perhaps also by the presence of enemies or the scarcity of proper food. The univalve and bivalve mollusca, of which the whelk and the cockle may be taken as types, move so slowly in their adult state, that we should expect them to have an exceedingly limited distribution; but the young of all these are free swimming embryos, and they thus have a powerful means of dispersal, and are carried by tides and currents so as ultimately to spread over every shore and shoal that offers conditions favourable for their development. The fresh water molluscs, which one might at first suppose could not range beyond their own river-basin, are yet very widely distributed in common with almost all other fresh water productions; and Mr. Darwin has shown that this is due to the fact, that ponds and marshes are constantly frequented by wading and swimming birds which are pre-eminently wanderers, and which frequently carry away with them the seeds of plants, and the eggs of molluscs and aquatic insects. Fresh water molluscs just hatched were found to attach themselves to a duck's foot suspended in an aquarium; and they would thus be easily carried from one lake or river to another, and by the help of different species of aquatic birds, might soon spread all over the globe. Even a water-beetle has been caught with a small living shell (Ancylus) attached to it; and these fly long distances and are liable to be blown out to sea, one having been caught on board the Beagle when forty-five miles from land. Although fresh water molluscs and their eggs must frequently be carried out to sea, yet this cannot lead to their dispersal, since salt water is almost immediately fatal to them; and we are therefore forced to conclude that the apparently insignificant and uncertain means of dispersal above alluded to are really what have led to their wide distribution. The true land-shells offer a still more difficult case, for they are exceedingly sensitive to the influence of salt water; they are not likely to be carried by aquatic birds, and yet they are more or less abundant all over the globe, inhabiting the most remote oceanic islands. It has been found, however, that land-shells have the power of lying dormant a long time. Some have lived two years and a half shut up in pill boxes; and one Egyptian desert snail came to life after having been glued down to a tablet in the British Museum for four years!
We are indebted to Mr. Darwin for experiments on the power of land shells to resist sea water, and he found that when they had formed a membranous diaphragm over the mouth of the shell they survived many days' immersion (in one case fourteen days); and another experimenter, quoted by Mr. Darwin, found that out of one hundred land shells immersed for a fortnight in the sea, twenty-seven recovered. It is therefore quite possible for them to be carried in the chinks of drift wood for many hundred miles across the sea, and this is probably one of the most effectual modes of their dispersal. Very young shells would also sometimes attach themselves to the feet of birds walking or resting on the ground, and as many of the waders often go far inland, this may have been one of the methods of distributing species of land shells; for it must always be remembered that nature can afford to wait, and that if but once in a thousand years a single bird should convey two or three minute snails to a distant island, this is all that is required for us to find that island well stocked with a great and varied population of land shells.
Means of Dispersal of Insects and the Barriers which Limit their Range.—Winged insects, as a whole, have perhaps more varied means of dispersal over the globe than any other highly organised animals. Many of them can fly immense distances, and the more delicate ones are liable to be carried by storms and hurricanes over a wide expanse of ocean. They are often met with far out at sea. Hawk-moths frequently fly on board ships as they approach the shores of tropical countries, and they have sometimes been captured more than 250 miles from the nearest land. Dragon-flies came on board the Adventure frigate when fifty miles off the coast of South America. A southerly wind brought flies in myriads to Admiral Smyth's ship in the Mediterranean when he was 100 miles distant from the coast of Africa. A large Indian beetle (Chrysochroa ocellata) was quite recently caught alive in the Bay of Bengal by Captain Payne of the barque William Mansoon, 273 miles from the nearest land. Darwin caught a locust 370 miles from land; and in 1844 swarms of locusts several miles in extent, and as thick as the flakes in a heavy snowstorm, visited Madeira. These must have come with perfect safety more than 300 miles; and as they continued flying over the island for a long time, they could evidently have travelled to a much greater distance. Numbers of living beetles belonging to seven genera, some aquatic and some terrestrial, were caught by Mr. Darwin in the open sea, seventeen miles from the coast of South America, and they did not seem injured by the salt water. Almost all the accidental causes that lead to the dispersal of the higher animals would be still more favourable for insects. Floating trees could carry hundreds of insects for one bird or mammal; and so many of the larvæ, eggs, and pupæ of insects have their abode in solid timber, that they might survive being floated immense distances. Great numbers of tropical insects have been captured in the London docks, where they have been brought in foreign timber; and some have emerged from furniture after remaining torpid for many years. Most insects have the power of existing weeks or months without food, and some are very tenacious of life. Many beetles will survive immersion for hours in strong spirit; and water a few degrees below the boiling point will not always kill them. We can therefore easily understand how, in the course of ages insects may become dispersed by means which would be quite inadequate in the case of the higher animals. The drift-wood and tropical fruits that reach Ireland and the Orkneys; the double cocoa-nuts that cross the Indian ocean from the Seychelle Islands to the coast of Sumatra; the winds that carry volcanic dust and ashes for thousands of miles; the hurricanes that travel in their revolving course over wide oceans; all indicate means by which a few insects may, at rare intervals be carried to remote regions, and become the progenitors of a group of allied forms.
But the dispersal of insects requires to be looked at from another point of view. They are, of all animals, perhaps the most wonderfully adapted for special conditions; and are so often fitted to fill one place in nature and one only, that the barriers against their permanent displacement are almost as numerous and as effective as their means of dispersal. Hundreds of species of lepidoptera, for example, can subsist in the larva state only on one species of plant; so that even if the perfect insects were carried to a new country, the continuance of the race would depend upon the same or a closely allied plant being abundant there. Other insects require succulent vegetable food all the year round, and are therefore confined to tropical regions; some can live only in deserts, others in forests; some are dependent on water-plants, some on mountain-vegetation. Many are so intimately connected with other insects during some part of their existence that they could not live without them; such are the parasitical hymenoptera and diptera, and those mimicking species whose welfare depends upon their being mistaken for something else. Then again, insects have enemies in every stage of their existence—the egg, the larva, the pupa, and the perfect form; and the abundance of any one of these enemies may render their survival impossible in a country otherwise well suited to them. Ever bearing in mind these two opposing classes of facts, we shall not be surprised at the enormous range of some groups of insects, and at the extreme localization of others; and shall be able to give a rational account of many phenomena of distribution that would otherwise seem quite unintelligible.