I stated above, that, although this diminution of stature is a very general accompaniment of isolation, amongst insects which have been _long_ cut off from the rest of their kind, there is no rule without an exception to it; and that, therefore, we must not always anticipate the result which has been described. We should remember that _immense_ periods of time are apparently necessary before any perceptible change can come over creatures from the stoppage of their migratory progress, and the unnatural in-breeding of their several tribes; so that in islands geologically recent (which often implies, however, their existence through epochs which would sound vast indeed to ears unscientific) we must not invariably expect to discover evidences of this law. On the contrary, we must first of all take into account the age of their formation, before we can judge _a priori_ as to the probability of its operation through a sufficient interval of time to have become conspicuous in its effects. I say "through a sufficient interval of time," because the process of deterioration may be silently going on, even now, in many an island, _which has not yet shown any matured traces of its action_, except perhaps in the case of a few species which appear to be more particularly susceptible to contingencies from without. We should then call to mind, that an enormous proportion of nearly every insular fauna is composed of accidental colonists during the last few centuries, in which civilization and commerce have been unintentionally at work in the cause of animal diffusion; and that, therefore, if modifications in outward contour have not necessarily resulted during a positive _geological_ interval, it would be absurd to look for them in the mere settlers (as it were) of yesterday.