have become abrupt from causes which are merely accidental_. Having now, however, examined the question in its broadest phasis, that is to say, on the supposition that Nature is _complete_ in her several links and parts; I shall perhaps be expected to offer a few passing words on what I have already hinted at,--namely, the possibility of genera being absolutely well-defined, even on their outer limits, _from accident_. Briefly, then, it is through the extinction of species that groups may, in some instances, be abruptly expressed: but, as such contingences are at all times liable (whether from natural or artificial causes) to happen; it would be unfair to build up our generic _definition_ from examples which are the exception, and not the rule,--and, _more_ than mere "exceptions" (as commonly understood by that term), the result of positive disturbances from without. Yet, that genera thus distinctly bounded, at either end, do actually occur, must be self-evident to any one who has attempted to study the distribution of organic beings with reference to the geological changes which have taken place on the earth's surface; for it is clear that a vast proportion of the creatures which inhabit our globe came into existence at periods _anterior_ to many of those great convulsions which altered finally the positions of sea and land, apportioning to each the areas which they now embrace: so that, if _generic provinces_ of radiation (no less than specific centres) be more than a fancy or romance, it is certain that numerous members of many geographical assemblages must have perished for ever during the gigantic sinkings which have at various epochs been brought about. From which it follows, _that those groups, or clusters, of which but few representatives (comparatively) are extant, will be more or less abruptly terminated, according as the original type to which they severally belong was peculiar, and in proportion as the number of its exponents has been reduced_. Although there are many means through which species may become annihilated, yet, since the subsidence of a tract into the sea involves the maximum of loss which a space of that magnitude can sustain, the above conclusion gives rise to a corollary: _that it is in islands that we should mainly look for genera which are to be rigidly pronounced_. The question therefore naturally suggests itself,--Is this in harmony with what we see; or, in other words, is it consistent with experience, or not? I believe that it is; for I think it will be found, on inquiry, _that the greater proportion of those groups which are more especially isolated in their character_ (I do not say, necessarily, the most anomalous; though this in some measure follows from the fact of their detachment) _are peculiar to countries which are insular_.