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Show 354 DARWINISM: CHAP. for beino- carried by mammalia or birds, and for floating in the water o~ throuo-h the air, while many are so small and so lio-ht that there b is practically no limit to the distances they b . may be carried by gales and ~urncan~s. . vV c may, therefore, feel qmte cm·tam that the · means of dJstribution that have enabled the larger mammalia to reach the most remote reo-ions from a common starting-point, will he n.t least as efficacio~s, and usually far more efficacious, with all other land animals and plants; and if in every case the existing distribution of this cla,ss can be explained on the theory of oceanic and continental permanence, with the limited changes of sea and land already referred to, no valid objections can he taken against this theory founded on anomalies of distribution in other orders. Y ct nothing is more common than for students of this or that group to assert that the theory of oceanic permanence is quite inconsistent with the distribntio11 of its various species and genera. Because a few Intlian o-encra and closely allied species of birds are found in Mad;t~ ascar, a land termed "Lemuria" bas been supposed to h :1Yc ~nited the two countries during a comparatively rccc11L geological epoch; while the similarity of fossil plants a1 1d reptiles, from the Permian and Miocene forma,tions of lnd ia and South Africa, has been adduced as further cvi<lence of th is connection. But there are also genera of snn.kes, of insects, and of plants, common to Madagascar and South America, only, which have been held to necessitate a direct land connection between these countries. These views evidently refute themselves, because any such land connections must have led to a far greater similarity in tho productions of the several countries than actually exists, and would besi.tlcs render altogether inexplicable the absence of all the chief types of African and Indian mammalia from Madagascar, awl its marvellous individuality in every department of the orgn.nie world.1 Powers of Dispersal as illustmted by Insular Organism::.. Having arrived at the conclusion that our existing oceans have remained practically unaltered throughout theTertiaryan(l Secondary periods of geology, and that the distribution of the 1 For a full discussion of this question, see Island L1je, pp. 390-420. XII GEOGRAPHICAL DIS'rRIBUTION OF ORGANI:::>.MS 355 mammalia is such as might h:wc been brought about by their known powers of dispersal, and by such changes of land and sea, :Ls h;we probably or certainly occurred, we arc, of course, restricted to similar cau es to explain the much wider and .-omotimes more eccentric di tribution of other classe. of animals and of plants. In doing so, we have to rely partly on direct evidence of dispersal, afforded by the land organisms that have been observed far out at ea, or which have taken refuge on ships, as well as by the periodical visitants to remote islands ; but very largely on indirect evidence, afforded by the frequent presence of certain groups on remote oceanic islands, which some ancestral forms must, therefore, h<tvC reached by transmission aero s the ocean from di tant lands. Biuls. These vary much in their powers of flight, and their capability of traversing wide seas and oceans. .Many swimming and wading birds can continuo long on the wing, fly swiftly, and have, besides, the power of rc ting safely on the surface of the water. These would hardly be limited by any width of ocean, except for the need of food ; and many of them, as the gulls, petrels, and di vcrs, fi ncl ahnnclance of food on the surface of the sea itself. These gronps have a wide eli ·tribution ac1'o s tho ocean. ; whil waclcrs-espe ially plovers, sandpipers, snipes, and herons-are equally cosmopolitan, travelling along the coasts of all the continents, and across the narrow seas which separate them. Many of these birds seem unaffected by climate, and as the organisms on which they feed are equally abundant on arctic, temperate, and tropical shores, there is hardly any limit to the range oven of some of the species. Land-birds are much more restricted in their range, owing to their usually limited powers of flight, their inability to rest on the surface of the sea or to obtain food from it, and their greater specialisation, which renders them le s able to maintain themselves in the new countries they may occa ionally reach. Many of them are adapted to live only in woods, or in marshes, or in deserts ; they need particular kinds of food or a limited range of temperature ; and they are adapted to cope only with the special enemies or the particular group of |