Thus, it will be perceived, how necessary it is to take every element and contingency into account before we venture to pronounce dogmatically on either the existence or non-existence of any physical law; and how cautious we should be of denying the legitimate operation of external influences in one region, because they would seem, _prima facie_, to be contradicted in another. It is surely more philosophical to endeavour to reconcile the two, by tracing out (as may frequently be done) some opposing principle in the latter, which shall enable us to understand the discrepancy, and to believe that the same action may be going on in both cases, but that in one of them it is either overruled by a greater controlling power than itself, or else has not had sufficient time to bring its fruits to maturity. If a proposition be true, we should recollect that it is _always_ so (under all the circumstances and conditions to which it is applicable); for, otherwise, it would be both true and false,--which is absurd: hence, _if_ my premises be true, that the general tendency of isolation is to diminish the stature of those insects which have become isolated; it follows that that tendency must remain, so long as there are no other special disturbing influences to absorb or neutralize it. "When any observation," says a writer of the last century, "hath hitherto constantly held true, or hath _most commonly_ proved to be so, it has by this acquired an established credit: the cause may be presumed to retain its former force; and the effect may be taken as probable, _if in the example before us there doth not appear something particular,--some reason for exception_[35]." Hence it is, that, even amongst the _opposite_ phaenomena which one island may occasionally present from those of another, I have often been able to recognize the working of a selfsame law; and clearly to detect, that it is not from _its failure_, in either instance, that contending results are brought about, but simply that some counteracting agent has been exerting its energy in the one case, so as to nullify what would have otherwise come to pass. The main object however of the present section being to show that a considerable amount of power is due to isolation itself, in regulating (after a long series of ages) the outward aspect of the insect tribes, it is not strictly necessary that we should so rigidly insist on deterioration of size as one of its primary consequences,--since (whether it be so or not) we are merely concerned here to demonstrate, that its influence, _in some shape or other_, is absolute and real.