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Show 358 DARWINISM CHAP. ino- nio·htjars buntino-s white-thron,ts, willow-wrens, cuckoos, ho~1se-~parro;vs, robi~s: wheatcars, a.nd blackbirds. These had probably crossecl from Somersetshire, and had they been caught by a storm the larger portion of them must have been blown out to ca.1 These f:wts cnahl us to account sufficiently well for th birds of oceanic i lands, the number and variety of which ar ' seen to be proportionate to their .fa~ilitics for reaching the island and maintaining themselves m It. Thus, though more birds yearly reach Berm'_lda tha~ the Azorc., the number of residents in the latter Isbnds IS much larger, due to the greater extent of the islands, their numbe~·, and the~r more varied surface. In the Gahtpagos tho land-birds are still mon' numerous, duo in part to their larger a,rca and greater proxi mity to the continent, but chiefly to the abscnc? of storms, so that the birds which originally reached the Islands have remained loner isolated and have developed into many closely a,llied speciesb adapted to the special con~itiom.:.. ~ll the species of the Gn,lapa,gos but one are pecul~ar to the. Jslands, while the Azores possess only one pecuhar spec10s, . n,n(l Bermuda none-a fact which is clearly due to the contmtwl immicrration of fresh individuals keeping up the purity of the b~eed by intercrossing. In the Sandwich Islan~s, whieh are extremely isolated, being more thn,n 20?? m1le f~·om any continent or large island, · we have a cond1t1on of th.mgs similar to what prevails in the Galapagos, the land -bmls, eighteen in number, being all peculi~r, and belonging, except one, to peculiar genera. These b1rds have probably all descended from three or four original types which reached the islands at some remote period, proba,bly by mea,ns of intcrvenin(J' islets that have since disappearccL In St. Helena we have ~ degree of permanent isola.tion which has pre vented any land-birds from reaching the island ; for <Llthough its distance from the continent, 1100 miles, is not . o grc:1 t as in the case of the Sa,nclwich Islands, it is situa.ted in an ocean almost entirely destitute of small islands, while its position within the tropics renders it free from violent stor~ s. Neither is there, on the nearest part of the coast of Afnca, a perpetua,l stream of migrating birds like that which 1 Report of the Brit. A soc. Committee on Migration of Birds during 1886. xu GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 359 supplies the innumerable stragglers which every year reach Bermuda and tho Azores. Insects. 'Vingccl insects have been mainly dispersed in tho same way as birds, by their power of flight, aided by violent or lono·continucd winds. Being so small, and of such low specif.c gravity, they arc occasionally carried to still greater distances ; and thus no islands, however remote, arc altogether without them. The eggs of insects, being often deposited in borings <>r in crevices of timber, may have boon conveyed Ion()' distances by floating trees, as may the larvre of those specie~ which feed on wood. Several cases have been published of insects coming on board ships at great distances from land; aml Darwin records having caught a large grasshopper when the ship was 370 miles from tho coast of Africa, whence the insect hacl probably come. In the Entomologists' Monthly JJ[agazine for Juno 1885, Mr. MacLachlan has recorded tho occurrence of a swarm of moths in the Atlantic ocean, from the log of the ship Pleione. The vc sel was homeward bound from Now Zealand, and in Lat. 6° 47' N., Long. 32° 50' W., hundreds of moths appeared about the ship, settling in numbers on the spars and rigging. The wind for four days previously had been very light from north, north-west, or north-cast, and sometimes calm. The northea. t trade wind occasionally extends to the ship's position at thn,t time of year. The captain adds that "frequently, in that p;u-t of the ocean, ho has had moths and butterflies come on board." The position is 960 miles south-west of tho Ca.po V crdo Islands, and about 440 north-east of the South American coast. The specimen preserved is Deiopeia pulchclla, a very common species in dry localities in the En. tern tropics, and rarely found in Britain, but, Mr. MacLachlan thinks, not found in South America. They must have come, therefore, from tho Capo Verde Islands, or from some parts of the African coast, a,nd must have traversed about a thousand miles of ocean with the assistance, no doubt, <>f a strong north-east trade wind for a great part of the dista,nce. In the British Museum collection there is a specimen of the same moth caught at sea during the voyage of the Rattlesnake, |
