The hypothesis that there is a characteristic rate of exponential radiation for the Insecta is tested for forty-nine of the most diverse family-level taxa in the Coleoptera, Diptera, and Hymenoptera. Data used are estimates of Recent species diversity and age of first appearance in the fossil record, as in the original statement of the hypothesis by Stanley (1979). There is no evidence of a characteristic radiation rate for the insects. A more exact method is proposed for identifying clades that are radiating exponentially. When applied to Stanley's data for the mammalian family Bovidae, it indicates that the Bovidae are not radiating exponentially at a characteristic rate. Future research might emphasize the reliable identification of clades that are radiating exponentially, prior to drawing conclusions about rates that might characterize higher taxa.

The biogeography of differences in average shape of the modern planktonic foraminifer Globorotalia truncatulinoides (d'Orbigny) exhibits systematic relationships to changes in the ocean's surface environment. Comparison of these shape changes, as they exist today in the Southern Hemisphere, with fossil shapes preserved in a Late Pleistocene record from the South Atlantic Ocean, shows that the biogeography of G. truncatulinoides ecophenotypes has changed markedly through time. Beginning at least 700,000 yr ago and continuing up to the present, there has been a gradual but clear migration of certain morphotypes of G. truncatulinoides toward lower latitudes. The history of this migration bears no simple relationship to the cyclic climatic changes that characterize the Late Pleistocene. We conclude that either (1) phenotypic variants of Gr. truncatulinoides reflect some previously unmeasured, gradually changing aspect of Late Pleistocene oceans, or (2) we are witnessing a gradual evolution of the environment preferences of G. truncatulinoides.

To the student of evolution, Malmgren and Kennett's (1981) detailed study of the Globorotalia inflata lineage demonstrates the continuity of the stratigraphic record and the abundance of planktic microfossils in deep-sea sediments. Their work recalls the prediction (Prothero and Lazarus 1980) that planktic microfossils might serve as the laboratory animals of evolutionary studies and indicates the potential value of fossils in determining ancestry. However, for theorists to judge the evolutionary import of Malmgren and Kennett's high-resolution morphometric study, broader consideration of lineage relationships is needed. They portray the G. inflata lineage as a set of five successional taxa in simple ancestor-descendant relationship. Trends, established morphometrically and claimed to be “one of the clearest examples of gradualism in any group of fossils” (Malmgren and Kennett 1981, p. 236), are shown to progressively transform a compressed shell with peripheral keel (G. conoidea) into a globose shell (G. inflata) without a keel. While the succession of taxa given by Malmgren and Kennett has been supported in other studies of western South Pacific sequences (e.g., McInnes 1965; Walters 1965; Kennett 1966; 1973; Jenkins 1971; Scott 1979; Hornibrook 1982), it is not established that the order indicates simple phyletic transformation without branching, as Malmgren and Kennett imply. Crucial evidence concerns the history and status of keeled populations which resemble the ancestral G. conoidea but occur in the Pliocene, associated with advanced members of the G. inflata lineage. These populations are not mentioned by Malmgren and Kennett, yet, if they are also descendants of G. conoidea, their occurrence strongly suggests that lineage divergence occurred. I wish to review evidence in support of this view.

We appreciate Dr. Scott's comments about our paper (Malmgren and Kennett 1981), but wish to clarify some aspects which we hope will help provide a better understanding about the Late Neogene evolution of the Globorotalia (Globoconella) group (Kennett and Srinivasan 1983). We also wish to dispute some of Scott's other criticisms. In effect, Scott's arguments have provided no reason to justify abandonment of our initial contention that this lineage represents one of the best documented cases of phyletic gradualism in any group of fossils.

We support Scott's notion that divergence of the lineage took place near the Miocene-Pliocene boundary, but this divergence is not relevant for our evolutionary study. In fact, the Early Pliocene form resembling Globorotalia conoidea was already recognized by one of us (J.P.K.) 10 yr ago (Kennett 1973). It was distinguished as G. cf. conoidea and not G. conoidea as stated by Scott. In our previous papers (Malmgren and Kennett 1981, 1982) we confined our attention to the evolution that occurred in the primary temperate stock (cool subtropical) water mass—the abundant and dominant forms of Globoconella.