OCR Text |
Show 356 DARWINISM CHAP. competitors among which ~hey have .been developed. Such birds as these may pass agam and agam to a new country, but arc never able to establish themselves in it; :1nd it is this organic barrier, as it is termed, rat~er than any physical barrier, which, in mn,ny cases, dctcrmmcs tho presence of a, species in one area and its absence from another. Vv e must always remember, therefore, that, although the presence of a species in a remote oc~anic island clearly . proves that its ancestors must at one t1mc have found thmr way there, tho absence of a species does not prove the contrary, since it al so may have reached the island, but have been unable to main tain itself, owing to the inorganic or organic conditions not bcinO' suitable to it. This general principle applies to all class~s of orO'anisms, and there are many striking illustrations of it. In the Azores there are eighteen species of lnncl-birds which arc permanent residents, but there arc al o several others which reach the islands almost every year after great storms, but have never been able to establish themselves. In Bermuda the facts are still more striking, since there arc only ten species of resident birds, while no less than twenty other species of land- birds and more than a hundred species of waders and aquatics are frequent visitor , often in great numbers, but are never able to esta,blish themselves. On the same principle we account for the fact that, of the many continental insects and birds that have been let loose, or have escaped from confinement, in this country, hardly one has been able 'to maintain itself, and the same phenomenon is still more striking in the case of plants. Of the thousands of hardy plants which grow easily in our gardcnH, very few have ever run wild, and when the experiment is purposely tried it invariably fails. Thus A. de Candollc informs us that several botanists of Paris, Geneva, and especially of Montpellier, have sown the seeds of many hundreds of species of exotic hardy plants, in what appeared to be the most favourable situations, but that in hardly a. single case has any one of them become naturaliscd.l Still more, then, in plants than in animals the absence of a species does not prove that it has never reached the locality, but merely that it has not been able to maintain itself in com- 1 Geographie Botanique, p. 798. XII GEOGRAPHICAL DISTRIBUTION OI? ORGANISMS 357 petition with the native productions. In other cases, as we have seen, facts of an exactly opposite nature occur. The ra.t, the pig, and the rabbit, the water-cress, the clover, and many other plants, when introduced into New Zealand, flourish cxceedin~ly, and even exterminate their native competitors; so that m these cases we may feel sure that the species in question did not exist in N cw Zealand imply because they hacl been unn,ble to reach that country by their natural means of dispersal. I will now give a few ca cs, in addition to those recorded in my previous works, of birds and insects which have been observed far from any land. Bird:; and Insects at Sea. Captain D. Fullarton of the ship 1'imam recorded iu his log the occurrence of a great number of small land-birds about the ship on 15th March 1 6, when in Lat. 4 o 31 ' N., Long. Bo 16' 'V. He says : "A great many smalllancl-bircls a bout us· · put about sixty into a coop, evidently tir d out." And tw~ days h~.tcr, 17th March, " Over fifty of the birds cooped on 15th ch.cd, though feel. parrows, finches, water-wagtails, two small bn·ds, name unknown, one kind like a linn t, and a large bird like a starling. In all there have been on board over seventy birds, besides some that hovered abont us for . orne time and then fell into the sea exhausted." Ea.·tcrly winds and severe weather were experienced at the timc. 1 The spot where this remarkable flight of birds was met with is about 160 miles clue west of Brest, and this is the least distance the hirds mu. t have hcen carried. It is interesting to note that the position of the ship is nearly in the line from the EtlO'lish and French coasts to the Azores, where, after great storm~, so many bird stragglers arrive annually. The. c birds were probably blown out to sea dming their :pring mi crration along the outh coast of England to \\ralcs and Irehn<l. Dnrino· the autumnal migrn,tion, however, great flocks of l>ird -cspe~ially starlings, thru. hes, and fielclfarcs-havc been observed every year flying out to sea from the west coast of Ireland, almost the whole of which must perish_ At the Nash Lighthou. c, in the Bristol Channel on the coast of Glamorganshire, an enormous number of small birds were observed on 3<.1 'cptembcr, inclu(l- 1 NatltTe, 1st April 1886. |