摘要:

SUMMARY1‘Classica’ classifications of Megascolecoid earthworms are reviewed and the assumptions on which they were based are described. Formerly morphological series were regarded as phylogenetic series and member species were believed to be ancestors and descendants living together contemporaneously. ‘Family‐tree’ type classifications were constructed on these sequences and the systems were regarded as ‘natural’, i.e. phylogenetic, classifications.2More recently Omodeo (1958), Lee (1959) and Gates (1959) separately rejected the concept of classifying new taxa by definition, that is, placing them in the appropriate places in a family‐tree according to the weighting given to single characters. Instead each proposed a new classification for the Megascolecoid earthworms based on overall similarities. The three classifications are incompatible.3Taxonometric methods are described by which a less subjective approach to the problem of Megascolecoid classification is possible by taking into account a relatively large number of attributes.4Thirty species, representing twenty‐nine genera of Megascolecoid earthworms were examined. Forty‐three attributes, eighteen quantitative and twenty‐five qualitative, were scored and the assessments computed to produce a matrix of similarity coefficients. The quantitatives were computed to permit comparisons to be made over the total range of the character states assessed (similar to the requirements of Cain&Harrison, 1958), while the qualitatives were treated on a match or mismatch basis.5The matrix of similarity coefficients was examined by two methods. Firstly, by clustering by single linkage, ‘Sneath's method’, to produce a dendrogram and secondly, by transforming the original matrix by ‘Gower's formula’ and calculating the latent roots and vectors of which the first and the second were employed as co‐ordinates in a vector diagram.6Two assemblages of genera could be detected in the dendrogram but they did not cluster in the vector diagram. Instead they formed a scatter with genera of one assemblage occupying one‐half of the diagram and genera of the other assemblage one quadrant of the remaining half. The fourth quadrant contained a third assemblage comprising genera which, due to their low similarity coefficients, could not be recognized in the dendrogram as having any special mutual similarity.7The pattern of arrangement of the genera was found to coincide largely with the classification previously proposed by Gates and to disagree with those proposed by both Omodeo and Lee.8It is concluded that: (a) Problems of oligochaete systematics are suited for study by computer techniques, (b) The results of newer taxonometric procedures need not be a variance with the results obtained by more traditional methods, (c) In this particular investigation, the classification proposed for the Megascolecoid earthworms by Gates (1959), has been confirmed. (The ‘key’ or ‘spot’ characters by which the component family groups may be recognized are firstly, the structure of the prostatic glands and secondl

ISSN:0370-0461    

DOI:10.1111/j.1095-8312.1966.tb00955.x

出版商:Blackwell Publishing Ltd    

年代:1966

数据来源: WILEY

ISSN:0370-0461    

DOI:10.1111/j.1095-8312.1966.tb00956.x

出版商:Blackwell Publishing Ltd    

年代:1966

数据来源: WILEY

ISSN:0370-0461    

DOI:10.1111/j.1095-8312.1966.tb00957.x

出版商:Blackwell Publishing Ltd    

年代:1966

数据来源: WILEY

摘要:

SUMMARYAn account is given of the main vegetation zones of Somaliland (Somali Republic, Northern Region). Brief accounts are also given of the geology, geomorphology, and the present and past climate. An attempt is made to determine with which meteorological factors the zonation is most closely correlated. A rainfall map is given and its close similarity to a map of the vegetation zones indicates a close correlation between vegetation and rainfall. There are, however, many factors preventing a direct relationship and they are discussed.Six principal zones are recognized between the sea‐shore and the forest on the high ground along the top of the escarpment. These zones may readily be arranged in order according to increasing rain and altitude, and decreasing temperature. In addition to these six zones there are two other major ones. One is described asGypsecms areasand it is clear that its vegetation is controlled by its unusual soil, and the other is theHand‐type mixed bush. The latter occurs on the plateau on reddish loamy sands which cannot be regarded as unusual soils; yet this vegetation type does not fit into the Somaliland succession as exemplified between the coast and the top of the escarpment. It is suggested that this extremely important zone, which extends south‐westwards for over 1000 km., is nevertheless determined largely by its soil which makes particularly good use of the available water supply.In each zone the vegetational changes that have occurred and are still in progress are indicated. The accounts of early travellers, together with the field evidence, indicate that the vegetation of Somaliland was already showing signs of overgrazing at the turn of the century, and that the rate of deterioration has increased since then. Stock numbers have continued to rise and the grazing pressure has increased in proportion, and it is clear that the rapidly worsening grazing situation is reaching the point of diminishing returns. Stock numbers are now at such a level that it is doubtful whether any system of grazing control or rotational grazing scheme could save the position unless it is preceded by a large‐scale stock‐reduction scheme.The tendency for the Somali to settle and take up agriculture as opposed to the nomadic way of life is discussed. The amount of land, as judged by suitability of soil and adequacy of rainfall, is limited. In addition to these favoured areas, which lie in the west and to the south of the main watershed, other areas could grow crops if adjoining unproductive slopes were used as catchment areas, copying the natural system found in vegeta

ISSN:0370-0461    

DOI:10.1111/j.1095-8312.1966.tb00958.x

出版商:Blackwell Publishing Ltd    

年代:1966

数据来源: WILEY