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Mark Twain once said that Hawaii is ... the loveliest fleet of islands that lies anchored in any ocean. Poets, of course, are always right. Thus, the islands are indeed anchored; they occupy a fixed position near the middle of the vast Pacific crustal plate (50). The latter, however, is adrift on the earth's mantle, edging northwestward about nine centimeters each year. The islands have evidently been formed one after another as this plate has moved over a very hot melting spot in the mantle. These circumstances add enormously to the value of the islands as a natural laboratory for evolutionary events. Between five and six million years ago the island now called Kauai (Figure 1) rested near the present location of what is now the newest island (Hawaii, frequently called the Big Island). Each newer island rose successively above the ocean, a process that culminated in a chain of islands, all slowly drifting northwestwardly together (Figure 2). Indications suggest that the Hawaiian archipelago, or at least its present major islands, is now and always has been isolated by more than 3500 kilometers of unbroken ocean from any other continent or island group. The terrestial biota thus descended from waifs which originally crossed the ocean by long-distance dispersal (8). Chance has played a key role in determining what did and what did not arrive in Hawaii. Stochastic events, however, were not confined simply to the first arrivals. As each new island appeared to the southeast of Kauai, there arose a new set of opportunities for the repeated operation of chance colonization. In order to appreciate the extraordinary nature of the terrestrial ecology of Hawaii, some other basic facts should be borne in mind. The chance arrivals, coming from all points of the compass, produced a discordant biota (8). Natural selection
Carson et al. (Mon,) studied this question.