The geographical distribution of animals, with a study of the relations ...
The geographical distribution of animals, with a study of the relations ... The geographical distribution of animals, with a study of the relations ...
32 DISPERSAL AND MIGRATION. [part i.times attach themselves to the feet of birds walking or restingon the ground, and as many of the waders often go far inland,this may have been one of the methods of distributing speciesof land shells ;for it must always be remembered that nature canafford to wait, and that if but once in a thousand years a singlebird should convey two or three minute snails to a distant island,thisis all that is required for us to find that island well stockedwith a great and varied population of land shells.Means of Dispersal of Insects a?id the Barriers luhich Limittheir Range.—Winged insects, as a whole, have perhaps morevaried means of dispersal over the globe than any other highlyorganised animals. Many of them can fly immense distances,and the more delicate ones are liable to be carried by stormsand hurricanes over a wide expanse of ocean. They are oftenmet with far out at sea. Hawk-moths frequently fly on boardships as they approach the shores of tropical countries, and theyhave sometimes been captured more than 250 miles from thenearest land. Dragon-flies came on board the Adventure frigatewhen fifty miles off the coast of South America. A southerlywind brought flies in myriads to Admiral Smyth's ship in theMediterranean when he was 100 miles distant from the coast ofAfrica. A large Indian beetle (Chrysochroa ocellata) was quiterecently caught alive in the Bay of Bengal by Captain Payne ofthe barque William Mansoon, 273 miles from the nearest land.Darwin caught a locust 370 miles from land; and in 1844swarms of locusts several miles in extent, and as thick as theflakes in a heavy snowstorm, visited Madeira.come with perfect safety more than 300 miles ;These must haveand as they continuedflying over the island for a long time, they could evidentlyhave travelled to a much greater distance, Numbers of livingbeetles belonging to seven genera, some aquatic and some terrestrial,were caught by Mr. Darwin in the open sea, seventeenmiles from the coast of South America, and they did not seeminjured by the salt water.Almost all the accidental causes thatlead to the dispersal of the higher animals would be still morefavourable for insects. Floating trees could carry hundreds ofinsects for one bird or mammal ;and so many of the larvre, eggs,
;chap, ii.] DISPERSAL AND MIGRATION. 33and pupse of insects have their abode in solidmight survive being floated immense distances.timber, that theyGreat numbersof tropical insects have been captured in the London docks,where they have been brought in foreign timber ;and some haveemerged from furniture after remaining torpid for many years.Most insects have the power of existing weeks or months withoutfood, and some are very tenacious of life. Many beetleswill survive immersion for hours in strong spirit ; and water afew degrees below the boiling point will not always kill them.We can therefore easily understand how, in the course of agesinsects may become dispersed by means which would be quiteinadequate in the case of the higher animals.The drift-wood andtropical fruits that reach Ireland and the Orkneys ; the doublecocoa-nuts that cross the Indian ocean from the Seychelle Islandsto the coast of Sumatra; the winds that carry volcanic dust andashes for thousands of miles ;revolving course over wide oceans ;the hurricanes that travel in theirall indicate means by whicha 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 fromanother point of view. They are, of all animals, perhaps themost wonderfully adapted for special conditions ;and are so oftenfitted to fill one place in nature and one only, that the barriersagainst their permanent displacement are almost as numerousand as effective as their means of dispersal.Hundreds of speciesof lepidoptera, for example, can subsist in the larva state only onone species of plant ; so that even if the perfect insects werecarried to a new country, the continuance of the race would dependupon the same or a closely allied plant being abundantthere. Other insects require succulent vegetable food all theyear round, and are therefore confined to tropical regionssome can live only in deserts, others in forests ; some are dependenton water-plants, some on mountain-vegetation.Manyare so intimately connected with other insects during somepart of their existence that they could not live without themsuch are the parasitical hymenoptera and diptera, and thosemimicking species whose welfare depends upon their being
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;chap, ii.] DISPERSAL AND MIGRATION. 33and pupse <strong>of</strong> insects have <strong>the</strong>ir abode in solidmight survive being floated immense distances.timber, that <strong>the</strong>yGreat numbers<strong>of</strong> tropical insects have been captured in <strong>the</strong> London docks,where <strong>the</strong>y have been brought in foreign timber ;and some haveemerged from furniture after remaining torpid for many years.Most insects have <strong>the</strong> power <strong>of</strong> existing weeks or months <strong>with</strong>outfood, and some are very tenacious <strong>of</strong> life. Many beetleswill survive immersion for hours in strong spirit ; and water afew degrees below <strong>the</strong> boiling point will not always kill <strong>the</strong>m.We can <strong>the</strong>refore easily understand how, in <strong>the</strong> course <strong>of</strong> agesinsects may become dispersed by means which would be quiteinadequate in <strong>the</strong> case <strong>of</strong> <strong>the</strong> higher <strong>animals</strong>.<strong>The</strong> drift-wood andtropical fruits that reach Ireland and <strong>the</strong> Orkneys ; <strong>the</strong> doublecocoa-nuts that cross <strong>the</strong> Indian ocean from <strong>the</strong> Seychelle Islandsto <strong>the</strong> coast <strong>of</strong> Sumatra; <strong>the</strong> winds that carry volcanic dust andashes for thousands <strong>of</strong> miles ;revolving course over wide oceans ;<strong>the</strong> hurricanes that travel in <strong>the</strong>irall indicate means by whicha few insects may, at rare intervals be carried to remote regions,and become <strong>the</strong> progenitors <strong>of</strong> a group <strong>of</strong> allied forms.But <strong>the</strong> dispersal <strong>of</strong> insects requires to be looked at fromano<strong>the</strong>r point <strong>of</strong> view. <strong>The</strong>y are, <strong>of</strong> all <strong>animals</strong>, perhaps <strong>the</strong>most wonderfully adapted for special conditions ;and are so <strong>of</strong>tenfitted to fill one place in nature and one only, that <strong>the</strong> barriersagainst <strong>the</strong>ir permanent displacement are almost as numerousand as effective as <strong>the</strong>ir means <strong>of</strong> dispersal.Hundreds <strong>of</strong> species<strong>of</strong> lepidoptera, for example, can subsist in <strong>the</strong> larva state only onone species <strong>of</strong> plant ; so that even if <strong>the</strong> perfect insects werecarried to a new country, <strong>the</strong> continuance <strong>of</strong> <strong>the</strong> race would dependupon <strong>the</strong> same or a closely allied plant being abundant<strong>the</strong>re. O<strong>the</strong>r insects require succulent vegetable food all <strong>the</strong>year round, and are <strong>the</strong>refore confined to tropical regionssome can live only in deserts, o<strong>the</strong>rs in forests ; some are dependenton water-plants, some on mountain-vegetation.Manyare so intimately connected <strong>with</strong> o<strong>the</strong>r insects during somepart <strong>of</strong> <strong>the</strong>ir existence that <strong>the</strong>y could not live <strong>with</strong>out <strong>the</strong>msuch are <strong>the</strong> parasitical hymenoptera and diptera, and thosemimicking species whose welfare depends upon <strong>the</strong>ir being
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