I alluded, in a previous section, to the _Dromius obscuroguttatus_, Dufts., as presenting permanent characteristics in Madeira,--the combined result of latitude and isolation; and I also stated that it was not always possible, whilst dealing with physical agents which are necessarily obscure, to refer the respective phaenomena (whatsoever they may be), which would seem to have departed from their types, to a single disturbing cause. Hence, whilst I there acknowledged latitude as in part answerable for the changes which that insect has undergone, I may here suggest that it is, in all probability, to _isolation_ that we must mainly look, if we would understand those changes aright. But what _are_ the distinctive features, it may be asked, which the _D. obscuroguttatus_ has adopted, since its first arrival from more northern latitudes over an unbroken[38] continent? It has not altered much, after all: it is, however, the _nature_ of the alterations, and their constancy, which give them their real importance. In a few words then, the insect is rather larger and more robust than its European analogue, and (to omit other minor differences) _its wings are evanescent_. But this, on our above hypothesis, is precisely what we should have expected: for, since it is self-evident that the species cannot have been naturalized accidentally on these mountains, and since geology informs us that a _vast_ interval has elapsed since the Madeiran islands were portions of a continuous whole, we have at once a sufficient _time_ assured us for the modifications to be completed, and to appear at length permanently adjusted in accordance with the conditions and influences which locally prevail.