modification is the normal one,--where the insect likewise displays the same lichenophagous tendency, and where the districts in which it exists are equally barren. But, if its maximum be attained in Madeira proper, and a certain number of minor deviations range throughout Porto Santo and the Dezerta Grande, it still remains for us to show where its _minimum_ is to be obtained:--which, true to the _modus operandi_ by which we have conjectured its divers degrees of abortion to have been brought about, would seem to be centred on the Northern Dezerta, or Ilheo Chao. When we bear in mind the minute dimensions of that flattened rock, which does not include so much as a single valley, or depression, within its bounds, and is consequently seldom free from the violence of the winds (which sweep across it incessantly, from whatever quarter they may arise); it could hardly be supposed that an insect which is so obviously subservient to atmospheric control should not have become materially affected, in its outward guise, through long seclusion on such a spot:--and accordingly we are not astonished to find the race which has been thus cut off for ages on this extraordinary little island, itself _as_ extraordinary. It is indeed very remarkable to trace out how clearly the agencies we are discussing have here operated on the species under consideration,--for both sexes (though especially the male) descend on the Ilheo Chao to somewhat less than half a line in length, being literally of scarcely greater magnitude than some of the larger representatives of the _Ptiliadae_!"[34]