But, however important an element, in the eradication of species, submergence may be; we must not entirely omit to notice other methods also, through the medium of which genera may become well-defined. We should recollect that the removal of a _very few_ links from an endemic cluster is sufficient to cause its disjunction from the type to which it is next akin, and that where the creatures which unite in composing it are of slow diffusive powers, or sedentary habits, the elimination of such links is (through the smallness of the areas which have been overspread) a comparatively easy operation. The accidental introduction of organic beings amongst others to the interests of which they are hostile, may be a powerful means, as Mr. Darwin has suggested, of keeping the latter in check, and of finally destroying them[83]. The gradual upheaval of a tract which has been well-stored with specific centres of radiation, created expressly for itself, may (through the climatal changes which have been brought about) succeed in extirpating races innumerable,--those only surviving which are able to adapt themselves to the altered conditions; and which would _now_ be consequently looked upon as abrupt topographical assemblages. The over-whelming effect of a volcanic eruption, in a region where the aborigines of the soil have not wandered far from their primaeval haunts, may, as Sir Charles Lyell has well remarked, put an end to others, and so effect the separation of their allies from the central stock. And, lastly, the intervention of man, with all the various concomitants which civilization, art, and agriculture bring in his train, is the most irresistible of every agency in the extensive (though often accidental) demolition of a greater or less proportion of the animate tribes.