Paul B. Sears

Ecology—a Subversive Subject

that ecology is merely a matter of emphasizing the obvious. It merely assembles matters of common knowledge and attempts to invest them with status by means of a special and not very winning vocabulary. The second represents an opposite extreme. Conceding the ultimate importance of understanding the great pattern of life and environment, sound logic and practice dictate that we must first get on with the infinitely detailed analysis of the many factors involved. In other words, we do not yet know enough about the bricks and mortar to get on with the building. I am certain these attitudes have been re-

PALYNOLOGY IN SOUTHERN NORTH AMERICA

Pollen analysis of two deep lacustrine cores under Mexico City indicates a series of moist-dry oscillations with longer trends of changing temperature—a climatic record which appears to extend as far back as early Wisconsin. The individual moist pulsations, although marked by warmth, were evidently times of glacial nourishment; the dry phases, some of which were cool, were evidently times of abatement. The upper 50 m of the profiles indicate two episodes of maximum glaciation; the details that follow the more recent agree with present knowledge of the Cary-Mankato-Latest sequence of the Wisconsin. Under milder conditions between the postulated Taze-well and Cary glacial maxima, soil might have formed on exposed till. The base of the profiles indicates warm-moist conditions with falling temperature to about 50 m. Because of tectonic and volcanic disturbance, it is not clear whether this cooling represents a separate advance or merely an early phase of the first maximum above the 50-m level. For the tropical Mexico City area, it is shown that the moist periods of glacial nourishment were also relatively warm.

PALYNOLOGY IN SOUTHERN NORTH AMERICA PART III: MICROFOSSIL PROFILES UNDER MEXICO CITY CORRELATED WITH THE SEDIMENTARY PROFILES

The sediments of the Cuenca de Mexico fall into two classes—transported materials from the surrounding highlands and residual materials deposited as a result of limnological processes, in situ . The latter include inorganic and organic constituents as well as microfossils. Studied together these components reflect events influenced or controlled by changing climates, by volcanic and tectonic activity, and by erosion whether natural or accelerated by man. Changes in percentage composition of upland forest pollen reflect changes in available moisture, and thus in climate. Changes in the density or abundance of recoverable pollen are closely related to changes in sedimentary zones and in water content of the sediments. High pollen density is associated with slow sedimentation and high water content of the sediments, whereas stages of rapid sedimentation and low water content are marked by sparseness or absence of pollen. As upland forest pollen composition is chiefly a function of climate, pollen density is a function of volcanic and other physiographic processes. Pollen and spores from lowland communities reflect swamp conditions and changes in vegetation caused by disturbance of habitat by volcanism, tectonic activity, and human occupation. Temporary disturbance from fresh ash fall and tectonic activity tends to be followed by a brief period of heavy density of ruderal pollen (composite, amaranth, grass) and by prompt restoration of the prevailing upland forest communities. Maize pollen was recovered throughout the archeological period and at a depth of 9 m, predating the presence of agricultural man in the basin. It was again recovered at 70m, a depth which represents considerable antiquity for maize; which is evidence in favor of the American origin of this plant. Contemporary pollen gathered from a site comparable to the Mexico City basin indicates relatively short distance of upland pollen transport. Therefore, the sparse spruce pollen found at various horizons in the Mexican cores probably came from the vicinity during times cooler than the present. Exceptionally heavy spruce pollen in samples from a recent horizon appears to be the result of redeposition of older sediments from the surrounding highlands. Correlation of the sedimentary components involves a complex of climatic, tectonic, volcanic and biotic factors; evidence from every possible source, in addition to conventional pollen analysis, is required for proper interpretation.

PALYNOLOGY IN SOUTHERN NORTH AMERICA. I: ARCHEOLOGICAL HORIZONS IN THE BASINS OF MEXICO

Profiles of sediments in the Basin of Mexico show an inverse relation between the percentages of fossil pine and oak pollen. Vegetation studies suggest that any marked increase in oak indicates an increase in available moisture, while consistently low percentages of oak indicate moisture deficiency. On this basis prolonged moist and dry periods have alternated. The more recent of these climatic shifts can be correlated stratigraphically with known phases of human occupation (Sears, 1951b). The Archaic Culture, 2000 or 1500 to 500 or 400 B.C., and the Nahua, 800 or 900 to 1521 A.D., both flourished within the Basin during relatively moist conditions. These two humid periods were separated by a prolonged dry interval which overtook the Late Archaic during a time of very low lake level. The Late Archaic level was then covered by a volcanic ash fall, and the culture shifted northward to higher ground. There it became the Teotihuacan which lasted from 500 or 400 B.C. to 800 or 900 A.D. Since this highly developed culture existed during a dry period, it must have made use of ground water. If so, the sources should have been unfavorably affected by heavy soil erosion known to have occurred, and by any deforestation, as Vaillant (1941) suggests in explanation of the ultimate Teotihuacan collapse. Conclusions based on pollen analysis are consistent with what is known of soil profiles and former lake levels.

Forest Sequences in the North Central States

1. Fifteen pollen profiles from bogs in the states of Illinois, Indiana, Michigan, and Ohio are figured, thirteen of which have not before been published. All but one are in the central deciduous region. 2. The forest sequence in these profiles shows two periods of retrogression. The earlier was a retrogression from fir and spruce to pine. This was followed by a relative increase of beech, ending in a second retrogression, when oak and hickory increased at the expense of beech. Three of the profiles show subsequent increase and retrogression of beech, but this does not appear to have been general. 3. The two general periods of retrogression are assumed to have been due to climatic causes, producing a less favorable water balance. This is considered adequate to explain the presence of xerothermic relicts in the central deciduous region.

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Vegetation, Climate, and Coastal Submergence in Connecticut.

Pollen analysis of a coastal marsh sediment at Guilford, Connecticut, indicates that there was a period of warmth and dryness preceding 3000 years ago. The subsequent increase of conifers at the expense of oak indicates a reversal that could account for decreasing rates of ice melt and rise of sea level. The record of herbaceous vegetation (grasses vs. sedges) suggests that the rise in the water table has been pulsating in character.

Variations in Cytology and Gross Morphology of Taraxacum. I. Cytology of Taraxacum laevigatum

1. The process of developmental selection is a normal event or succession of events in the life cycle of vascular plants, where it assumes various forms, being represented chiefly by embryonic selection, gametophytic selection, and gametic selection. 2. Developmental selection differs materially from natural selection, germinal selection, the intraselection of Roux, as well as the other selection theories. 3. Records of conspicuous cases of polyembryony in ferns are brought together. Original studies are added, constituting definite evidence that a selective plurality of embryos may normally exist even in the leptosporangiate ferns. Nearly all living ferns seem to have embryonic selection, or show evidence of having passed through a stage in which embryonic selection was the normal condition. The embryonic selection represented by the polyembryony of gymnosperms was derived from an embryonic selection habit in their fern ancestors. 4. Developmental selection in gymnosperms and angiosperms is not only represented by a selection among embryos, but also by a selection between female gametophytes and the male gametophytes represented by the pollen tubes. 5. A form of selection intermediate between natural selection and developmental selection may be recognized in the competition between buds and branches of a sporophyte or a branching thallus. 6. Developmental selection is a process which brings into play a definite internal competition between embryonic diploid individuals, as well as between the haploid sperms of fern plants, and the haploid male and female gametophytes of gymnosperms and angiosperms. On the other hand, natural selection usually acts on the diploid generation in these plant groups, or on the haploid fern gametophytes, where selection may take place in the external environment. 7. The discussion seeks to show why the process of developmental selection is not open to the more serious objections which have been urged against natural selection, and on what basis it equals or excels the latter as an effective selective process. 8. The discussion also shows how developmental selection may account for some of the phenomena of orthogenesis on a mechanical basis. 9. Developmental selection is not responsible for the origin of the chromosomal or other intracellular phenomena involved in mutation, but it is a powerful mechanism whose censorship may determine whether or not any particular intracellular phenomena causing mutation may complete the life cycle to be heritable, and therefore recognizable as a mutation.

PALYNOLOGY AND THE CLIMATIC RECORD OF THE SOUTHWEST

The southwestern United States contrasts strikingly with Scandinavia, where the statistical analysis of sedimentary pollen and other spores was first used as a clue to past vegetation and climate. In humid, glaciated northern Europe peat deposits are abundant, and they serve as an admirable preservative for these microfossils. Differences in bog strata and their organic inclusions, as well as certain facts of plant distribution, had long since aroused interest in post-Glacial climatic fluctuations. Thanks largely to pollen analysis the major fluctuations are now fairly clear and are known to correspond in general with episodes in North America. The warm and more or less arid conditions terminating it were reversed a t least twice: by the Port Huron readvance, circa 12,000 to 13,000 B.P., and by the Valders, circa 11,000 B.P., with the Two Creeks interval, circa 11,500 B.P., between them. The Valders was followed by a prolonged period of warming and desiccation, relieved by the humid Atlantic about 5000 to 6000 B.P. Cooler and moister conditions were initiated around the beginning of the Christian era, and fluctuations have occurred since that await further study. The challenge of climatic history in arid regions far beyond the limits of continental glaciation is even greater. Old terraces and lakes, dry or saline, along with numerous evidences of human activity, historic and earlier, suggest less arid conditions in the past. However, peat is scarce or absent except at high altitudes, while lake sediments are strongly inorganic. Accordingly, for a long time the most effective clue has been the record of tree-ring patterns, cleverly extrapolated back to about the beginning of the Christian era. This tree-ring analysis, begun by A. E. Douglas, had the double incentive of archaeological and astronomical interest, since the question of periodicity in solar behavior is an important one. However, like studies of post-Glacial peat, it deals with a limited and recent time span. On the other hand, any record that might be recovered from sedimentary basins in the Southwest would not have been interrupted by the physical presence of glaciers in these basins, and so might be expected to be more or less continuous back through the Pleistocene, or even beyond. If available, it would indicate something of the extent and intensity of Pleistocene climatic changes far outside the limits of continental glaciation, assist in correlating the numerous surface features associated with Pleistocene events in the region, and contribute to our understanding of biogeography. In addition, because of its continuity, such a record might eventually furnish data needed to investigate the principles as well as the pattern of climatic change. The possibility of using pollen analysis in the Southwest was first evident from sediments collected by Antevs in 1935 to 1936 and studied a t Oklahoma. These sediments came from temporary alluvial lakes in the Tsegi River Basin The last major ice advance took place 16,000 to 18,000 years ago.

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