摘要:

Summary1. Biological revolutions at major stratigraphical boundaries have been given numerous explanations involving endogenous biological, exogenous ecological, physical, and cosmic, as well as sedimentary or chemical factors. In an attempt to elucidate the true nature of these faunal revolutions and to assess the possible influence of biological and/or physical factors, the evolution of ammonites at the boundaries of Mesozoic stratigraphical Systems is reviewed. It is believed that the more detailed data now available can give a clearer impression of evolutionary events at these boundaries.2. It can be demonstrated that there is neither an abrupt and world‐wide extinction, nor a spontaneous replacement by new elements at these caesuras as had been generally supposed to have occurred at the Triassic‐Jurassic boundary, for example. Instead, one can recognize three distinct phases in the sequence of events: (1) a continuous disappearance of the ‘antique’ faunal elements; (2) a similarly continuous, gradual, and largely synchronous appearance of, or substitution by, qualitatively distinguishable ‘modern’ elements in small populations, yet in various parallel lineages (mosaic evolution); (3) a quite revolutionary, and quantitatively very sudden, diversification of these new elements, occurring at or with some delay above the boundary.3. Thus one can demonstrate both continuous evolution of the modern faunas (‘preadaptational phase’), as well as ‘discontinuous’ spontaneous revolution, which does not produce qualitatively new characters and must be explained by diversification or adaptive radiation. This means that no further explanation by internal factors or by higher mutation rates resulting from the impact of cosmic rays becomes necessary. It is believed that, preceded by high extinction rates, world‐wide ecological factors promoting higher niche diversity suffice to explain these adaptive radiations. The high degree of provincialism, endemism and specialization of the ‘antique’ faunas and the constant survival of smooth oxycones — regarded as inhabitants of a deep‐sea environment — demonstrate that marine regressions and transgressions were the most effective ecological factors.4. If there is not too much time involved between the two events, the caesura (Faunenschnitt) between final extinction of the old faunas and the radiation of the new is the most appropriate point by wh

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1973.tb00979.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

Summary1. The electrical and ionic gradients across a cell membrane depend on its permeability properties, on the concentration and net valency of the organic constituents of the cytoplasm and on the critical energy barrier to the extrusion of sodium. Such considerations do not, however, explain the small extent to which the concentration of potassium varies in myoplasm which may, instead, be related to the effects of potassium on particular enzymes.2. The fact that the apparent optimum level of potassium cannot usually be maintained in animals in which the extracellular level of sodium is below about 140 mM may explain why so many non‐marine animals have internal media of about that concentration, for more concentrated body fluids would require more work for their regulation.3. In axoplasm, the concentration of potassium is more nearly proportional to the concentration of sodium in the internal medium and this may partly explain the general correlation between the extracellular levels of sodium and potassium.4. The relation between pH and temperature in poikilothermic vertebrates is such as to suggest that the prime function of acid‐base regulation is to control the ionization of imidazole groups.5. High tensions of carbon dioxide cannot be maintained in water‐breathing animals because of the high solubility of this gas in water as compared with oxygen. Bicarbonate levels are correspondingly low to give a suitable pH. Higher tensions are possible in air‐breathing animals, and also necessary if water and heat are to be conserved, but an uncertain upper limit is set by the need for oxygen. The associated higher levels of bicarbonate confer the advantage of better buffering.6. Calcium and bicarbonate levels are not obviously limited by the solubility of calcium carbonate and a more general limitation on the composition of body fluids seems to arise from the low solubilities of calcium phosphates.7. The pattern of ionic balance in vertebrate plasma, reflected in a nearly constant value to the molar ratio ([Ca] + 5 × 10‐‐4)/([K] +0.034 [Na]), may be explained in terms of the maintenance of a constant electrical gradient across certain areas of cell membrane, between the inner and outer double layers.8. The patterns of cation balance in the haemolymphs of molluscs, crustacea and insects are also reviewed, with emphasis on the correlations existing between the concentrations of different cations. An attempt is made to relate the correlations in the mollusca to the concentrations of cations at the surfa

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1973.tb00980.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

Summary1. A review is presented of the information relating to the distribution of free‐living terrestrial Cryptostigmata, Prostigmata and Collembola in the south polar region.2. The Maritime zone, comprising the Antarctic Peninsula and its neighbouring islands, forms a less‐clearly definable faunal province than does the Continental zone where generic and specific endemism is high. The Maritime distribution of the micro‐arthropods forms no consistent pattern, although in the case of the two groups of mites a distinction can be made between a southern and a northern Maritime fauna. The boundary between these two elements appears to lie between the South Orkneys and the South Sandwich Islands.3. As far as distribution in the Sub‐Antarctic is concerned, all three groups of micro‐arthropods show certain general similarities, although in each case particular features can also be distinguished. All three show relatively high specific endemism in the Sub‐Antartcic, which sugge:ts that this zone is a faunal province distinct from that of the cold temperate zone to the north and other parts of the Antarctic to the south. It seems equally reasonable to recognize that this province can be subdivided into western and eastern parts, each with its own distinct group of species, although the extent to which this separation is expressed varies from group to group. In all three groups there is an element which is circum‐Sub‐Antarctic in distribution, but this element is more in evidence in the Collembola than in the mites. From the study of the distribution of endemic species in the Sub‐Antarctic it is concluded that the fauna of the eastern part has been isolated from the south temperate zone fauna for longer than it has in the west.4. In all three groups of micro‐arthropods a relic element can be identified, the present distribution of which is consistent with the idea of a former continuous distribution extending across a southern land mass incorporating both the eastern and western parts of the continent, the islands on the Scotia Ridge, and the older Sub‐Antarctic islands of South Georgia, Macquarie and, possibly, Kerguelen. In the Cryptostigmata in particular, this relic element, which is represented by members of the Podacaridae, is joined in the Sub‐Antarctic and Maritime zones by a penetrant element which probably has invaded the south polar region from the north during the post‐Pleistocene period.5. An alternative hypothesis is also discussed, namely that present distribution patterns of micro‐arthropods can be explained solely in terms of post‐Pleistocene colonization. It is accepted that limited overseas dispersal could have occurred in recent times, indeed may still be occurring between, for example, the various volcanic islands in the eastern Sub‐Antarctic, between the south temperate zone and the Sub‐Antarctic, and between the latter and the Maritime zone. However, there is no evidence to suggest that any appreciable amount of long‐range dispersal is occurring, on the scale required to support this hypothesis. In addition, there is no evidence that rates of speciation in the south polar region are rapid enough to produce the kind of evolutionary divergence which is implicit in the theo

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1973.tb00981.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

Summary1. Since use of the terms, symbiosis and obligate parasitism, is ambiguous, an attempt is made to re‐impose precision by a re‐evaluation of basic concepts.2. For symbiosis, there should be a return to a concept closer to that of de Bary's original, embracing those parasitic and mutualistic associations which involve permanent, intimate contact.3. Culturability should be abandoned as a criterion for classifying fungal behaviour.4. Instead of culturability, emphasis should be placed on the ecological and nutritional behaviour of fungi, permitting the recognition of five groups: obligate saprotrophs, facultative necrotrophs, obligate necrotrophs, facultative biotrophs and obligate biotrophs.5. Since environmental conditions can determine whether the nutrition of a fungus is biotrophic or necrotrophic, a scheme, speculative but amenable to experimental test, for the origin of biotrophy is proposed. This involves the interplay of alterations to patterns of translocation produced by fungally‐induced changes in hormonal balance in infected plants and catabolite repression of degradative enzymes of the fungus.6. The dependence of mutualistic symbiosis on the maintenance of biotrophy is stressed.7. Based on the nutrition of the diverse kinds of fungi involved in mycorrhizas, a re‐alignment of groupings of mycorrhizal associations is proposed. This directs research towards seeking generalizations within, and differences between, four clearly defined groups (sheathing, vesicular‐arbuscular, orchidaceous and ericaceous) instead of, as at present, within and between two (variously termed ectotrophic and endotrophic or ectomycorrhiza and endomycorrhiza), the second of which is highly artificial.Earlier drafts of the paper were read by Professors J. L. Harley, L. B. Thrower and S. D. Garrett, and Drs M. J. Carlile, B. C. Clifford, G. Hadley, P. M. Holligan, D. M. Losel and D. C. Smith. I am most grateful for their advice on specific points. Many, but not all, of their comments have been incorporated and I must stress that some views expressed, particularly with regard to the classification of mycorrhizas, are at variance with those of some of these c

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1973.tb00982.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1973.tb00983.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY