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Show 278 · ISLAND LTFK (PART II. various epochs by arms of Lhe sea uniting tbe two oceans across what is now Central America (the last separation being of recent date, as shown by the identical species of fishes on both sides of the isthmus), and the influence of the glacial epo~h in driving the temperate American flora south ward along the mountain plateaus.1 At the time when the two oceans w.ere united a portion of the Gulf Stream may have been diverted into the Pacific, giving rise to a current, some part of which would almost certainly have reached the Ga1apagos, and this may have helped to bring about that singular assemblage of West Indian and Mexican plants now found there. And as we now believe that the duration of the last glacial epoch in its successive phases was much longer than the time which has elapsed since it finally passed away, while throughout the Miocene epoch the soow-line would often be lowered during· periods of high excentricity, we are enabled to comprehend the nature of the causes which may have led to the islands beino· stocked 0 with those northern or sub-alpine types which are so cbar-acteristic a feature of that portion of the Galapa.gos flora which consists of peculiar species. On the whole, the flora agrees with the fauna in indicatin()' a moderately remote origin, great isolation, and changes of con~ ditions affording facilities for the introduction of organisms from various parts of the Americn.n con.st, aud even from the West Indian Islands and Gulf of Mexico. As in the case of the birds the several islands differ considerably in their native plants, man; species being limited to one or two islands only, while others extend to several. This is, of course, what might be expected on any theory of their origin; bec<tuse, even if the whole of the islands had once been united and afterwards separated lon()' cont1.nued isolation would often lead to the differentiation of' species, while the varied conditions to be found upon islanus differing in size and altitude aS well as in luxuriance Of veo·et<:Ltion, would oft2n lead to the extinction of a spe;.,ies on one i;laml antl its preservation on another. If the several islands had been equally well explored, it might be interesting to sec whether, as in the case of the Azores, the number of species diminished in 1 Geor;J·aphical Di.sf1·ibution of A11imnls, Vol. H. p. 81. C'IJAT'. Xllr.] TilE GALAPAGOS lSLANP~. 297 those more remote from the coast; hut unfortunately our knowledge of the productions of the various islands of the group is exceedingly unequal, aml, except in those cases in which representative spec·ies inhabit distinct islands, we have no certainty on the subject. All the more interesting problems in geographical distribution, however, arise from the relation of the faunn and flora of the group as a whole to those of the surrounding continents, and we shall therefore for the most part confine ourselves to this aspect of the question in our discussion of the phenomena presented by oceanic or continental islands. Conclnding Ren~aTks.-The Galapagos offer an instructive contrast with the Azores, showing how a difference of conclitions that might be thought unimportant may yet produce very striking results in the forms of life. Although the Galapagos are much nearer a continent than the Azores, the number of species of plants common to the continent is much less in the former case than in the latter, and this is still more prominent a characteristic of the insect and the bird faunas. This difference has been shown to depend, almost entirely, on the one archipelago being situated in a stormy, the other in a calm, portion of the ocean; and it demonstrates the preponderating importance of the atmosphere as an agent in the dispersal of birds, insects, and plants. Yet ocean-currents and surface-drifts are undoubtedly efficient carriers of plants, and, with plants, of insects and shells, especially in the tropics; and it is probably to this agency that we may impute the recent introduction of a number of common Peruvian and Ohilian littoral species, and also at a more remote period of several West Indian types when the Isthmus of Panama was submerged. In the case of these islands we see the importance of taking past conditions of sea and land and past changes of climate into account, in order to explain the relations of the peculiar or endemic species of their fauna and flora ; and we may even see an inJication of the ef-fects of climatal changes in the northern hemisphere, in the north temperate or alpine affinities of so many of the plants, and even of some of the birds. The relation between the migratory habits of the birds and the amount of difference from continentnl types is strikingly accordant with the fact that |