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Show 428 DARWINISM OllAP. elements, and in this way may profoundly modify the whole organisation. Why and how the external effects arc limited to special details of the structure we do not know j but it does not seem as if any far-reaching conclusions as to the cumulative effect of external conditions on the higher terrestrial animals and plants, can be drawn from such an exceptional phenomenon. It seems rather analogous to those effects of external influences on the very lowest organisms in which the vegetative and reproductive organs are hardly differentiated, in which case such effects are doubtless inherited.1 P1'ojessor Geddes's Them·y of Variation in Plants. In a paper read before the Edinburgh Botanical ~ ociety in 1 R 6 Mr. Patrick Geddes laid down the outlines of a fundamental theory of plant variation, which he has further extended in the article "Variation and Selection " in the Encyclopcedia Britwmica, and in a paper read before the Linnrean Society but not yet published. A theory of variation should deal alike with the origin of specific distinctions and with those vaster differences which characterise the larger groups, and he thinks it should answer such questions as-How an axis comes to be arrested to form .a flower ? how the various forms of inflorescence were evolved 'I how did perigynous or epigynous flowers arise from hypogynons flowers ? and many others equally fundamental. Natural selection acting upon numerous accidental variations will not, ho urges, account for such general facts as these, which must depend on some constant law of va,riation. This law he believes to be the well-known antagonism of vegetative aml reproductive growth acting throughout the whole course of plant development j and he uses it to explain many of the most characteristic features of the structure of flowers atHl fruits. 1 J n Dr. Weismann's essay on "Heredity," already referred to, be consi<lers it not improbable that changes in orgaui. ms produced by climatic influences may be inherited, because, as these changes do not affect the extern al part~ of n.n organi. m only, but often, as in the case of warmth or moisture prrmen. te the whole structure, they may possibly modify the germ- plasm itself, and thus induce variations in the next generation. In this way, he thinks, may possibly be explaiued the climatic varieties of certain butterlli es, and some other changes which seem to be effected by change of climate in a few generations. XIV FUNDAMENTAL PROBLEMS 429 Commencing with the origin of the flower, which all b?tanis~s agree in regarding as a shortened branch, he explams th1s shortenina as an inevitable }Jhy iological fact, since the cost of b . the development of the reproductive elements 1s so great as necessarily to check vegetative growth. In the same manner the shortening of the inflorescence from raceme to spike or umbel and thence to tho capitulum or den e flower-head of th~ composite plants is brought about. This shortening, carried still further, produces the flattened leaf-like receptacle of Dorstenia, and further still the deeply hollowed fruity receptacle of the fig. The flower itself undergoes a parallel modification due to a similar cause. It is formed by a series of modified leaves arranged round a shortened axis. In its earlier stage the number of these modified leaves is indefinite, as in many Ranunculacere j and the axis itself is not greatly shortened, as in Myosurus. The first advance is to a definite number of parts and a permanently shortened axis, in the ar~·ang~~ent termed hypogynous, in which all the whorls are qmte chstmct from each other. In the next stage there is a further shorteninO' of the central axis, leaving the outer portion as a ring on which the petals are inserted, producing the arrangement termed perigynous. A still further advance is made by the contraction of the axis, so as to leave the central part forming the ovary quite below the flower, which is then termed epigynous. These several modifications are said to be parallel and definite, and to be determined. by the continuous chccki11g of vegetation by reproduction along what is an absolute groove of progressive change. This being the case, the importance. of natural selection is greatly diminished. Instead of selectmg and accumulating spontaneous indefinite variations, its function is to retard them after the stage of maximum utility has been independently reached. The same simple conception is said to unlock innumerable problems of vegetable morphology, large and small alike. It explains the inevitable development of gymnosperm into angiosperm by the che~ked vege~tive growth of the ovule-bearing leaf or carpel j wh1le such mmor ad~ptations as the splitting fruit of the geranium or the cupped stigma of the pansy, can be no longer looked upon as achievements |
