Darwin's finches, the Geospizinae of the Galapagos, form a little world of their own, as Darwin commented. The important point is that the factors involved in their miniature adaptive radiation seem to be the same as, those involved in the larger world, so that they help to demonstrate evolutionary and ecological principles with unusual clarity. The chief difficulty is that, with only 13 species, statistical treatment may be dubious. The aim of the present paper is to review two problems discussed recently by other workers, namely the factors influencing differences between the different islands in (i) the degree of subspecific endemism and (ii) the number of resident species, particularly in relation to the possible roles of (a) isolation, and the associated difficulties of dispersal between the islands and (b) ecological differences in the vegetation and in the other finch species present. For this, it is necessary to have a reliable classification and a reliable assessment of island distributions, and I found it necessary to revise previous publications (including my own) in both respects, as set out in Table 1, with critical notes in Appendix 1. I hope that this revision may also be of value to other workers, who so often nowadays use Darwin's finches as a textbook example of evolution. It is not my aim here to discuss other evolutionary problems presented by the finches, nor, in particular, the ways in which Bowman (1961, 1963) differed from me (Lack, 1945, 1947), for which see also Bock (1963) and Selander (1962). In regard to the first of the two problems, I earlier concluded that the number of endemic subspecies of Darwin's finches on each island is correlated with the extent of its isolation from other islands in the archipelago. This view was rejected by Bowman (1961, p. 282-3), who claimed, instead, that the primary factor involved is the degree of difference in the vegetation on different islands, to which various subspecific differences in the finches are adapted. In a later mathematical analysis, Hamilton and Rubinoff (1963, 1964) upheld the importance of isolation and showed that the critical factor is the distance of the nearest other island. In regard to the second problem, the number of finch species on each island, Bowman (1961, p. 271-3) concluded: "the various Galapagos islands are so close to one another that irregularities in the distribution of the species of the Geospizinae are probably better attributed to the ecological unsuitability of the islands than the failure of dispersal"; and his discussion showed that the ecological factors which he had in mind concern differences in the vegetation on different islands. Later Hamilton and Rubinoff (1963, 1964, 1967) argued that the major factor involved is the degree of isolation of each island, as measured by the average distance of all the other islands; in addition (1964, correcting 1963), they concluded that its altitude (elevation) and the number of plant species present have some influence. The first problem, that of subspecific endemism, is relatively simple, and the seemingly divergent views of Bowman, stressing ecological differences, and of Hamilton and Rubinoff, stressing isolation, are both right, their views being complementary, not contradictory. The problem of what determines the number of species, raised in general by Hutchinson (1959) and excitingly developed for island life by MacArthur and Wilson (1963, 1967), is more complex. The two latter authors