imals has strengthened some parts of those animals and diminished the use of others, producing modifications that were then inherited by subsequent generations. He then extrapolates from this to nature in general, always acknowledging limitations in his method:
Under free nature, we can have no standard of comparison, by which to judge of the effects of long-continued use or disuse, for we know not the parent-forms; but many animals have structures which can be explained by the effects of disuse (Darwin, 1964, 134).
Darwin's method is to examine changes based on relationships between one generation and the next or one species and related species. He is explaining differences which can be observed, showing that the way a difference develops is based on the way the species responds to its environment:
When a part has been developed in an extraordinary manner in any one species, compared with the other species of the same genus, we may conclude that this part has undergone an extraordinary amount of modification, since the period when the species branched off from the common progenitor of the genus . . . An extraordinary amount of modification implies an unusually large and long-continued amount of variability, which has continually been accumulated by natural selection for the benefit of the species (Darwin, 1964, 153).
Such variations as are produced through these processes occur all the time, but the explosions in species that occur from time to time must have some other force causing the process of selection to take place at a higher rate. Adaptive radiations are described as a rapid and erratic series of bursts in species development, with a proliferation of changes from a
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