Nestling growth was studied in six species of Darwin's finches, and in two subspecies of one of them, on four islands of the Galapagos archipelago. The results show how the different beak and body size proportions among the adults of the species are brought about in ontogeny. Large species lay large eggs which give rise to large hatchlings, and in general these have higher growth rates than do the nestlings of smaller species. Geospiza difficilis hatches at a proportionally larger size than the other species and then grows at a slow rate. Variation with species tends to decline with nestling age, but this may be partly artefactual. Adult levels of variation are reached by about day 9, but in G. magnirostris bill depth variation increases significantly from day 9 to adulthood owing to differential mortality or growth. Bill depth variation is heritable in G. conirostris, the best-studied species, on every day from day 2 to day 10; heritable variation was demonstrated towards the end of the nestling phase of growth of this species in bill width, tarsus length and mass but not in bill length. In terms of relative growth, i.e. variates in proportion to body mass, there is only minor variation among species in growth rates of wing and tarsus but substantial variation in relative growth of bill dimensions. By extrapolation, this is one source of indirect evidence that pre-hatching growth differs among the species. The other evidence is that bill proportions differ at hatching relative to final adult size among species. In a multiple discriminant function analysis, the best discrimination among the three Geospiza species on Isla Genovesa was achieved by bill dimensions at all stages throughout the nestling period. Rates of relative growth were fastest in those bill dimensions that are most pronounced in adults. Each species changes in beak shape during nestling growth in a unique manner, except that G. difficilis on I. Genovesa and G. fuliginosa on I. Marchena, which are convergent as adults, have the same growth trajectories. The two subspecies of G. difficilis also have the same growth trajectories, and the differences in adult proportions are the allometric consequences of different stopping points along these trajectories. After fledging there is more growth remaining in bill length than in bill depth and width, but G. magnirostris is unique in first growing relatively quicker in bill depth, then in bill length. These results show that the adaptive differentiation of Darwin's finch species in adult bill morphology and size has been brought about by evolutionary changes in embryonic, nestling and post-fledging growth characteristics. A more comprehensive theory of the differentiation than exists at present could be developed by integrating ontogeny and phylogeny.