INSECTA MADERENSIA. 269 



however on the Dezerta Grande, and answering to the car. y. of the above diagnosis), 

 we immediately perceive that both of our requii-ed results are indicated, — the 

 reduction not being limited to size, but extended also to hue. In Porto Santo 

 this 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 Madeu-a proper, and a certain 

 number of minor delations range throughout Porto Santo and the Dezerta 

 Grande, it still remains for us to show where its m'mhmim 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 ^vinds (Avhich sweep across it incessantly, from whatever qviarter they may 

 arise) ; it could hardly be supposed that an insect which is so obviously subser- 

 \dent to atmospheric control should not have become materially affected, in its 

 outward guise, through long seclusion on such a spot : — and accordingly Ave are 

 not astonished to find the race which has been thus cut off for ages on this extra- 

 ordinary 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 FtiUadce ! 



After an accurate examination of a great mass of specimens of the P. alboplctus, 

 collected in dissimilar quarters and at nmnerous elevations, fom- principal phases 

 are all that I have been able to detect, — and which it will be perceived are mainly 

 dependent on geographical causes. To register every intervening gradation would 

 be superfluous ; nor, practically, could any advantage ensue from such a step, 

 since the very existence of varieties presupposes, from the nature of the case, the 

 media wliich are requisite to unite them to their parent type, — for, were such 

 indeed absent, we could have no warrant in pronouncing them to be varieties at 

 all. The utmost therefore that we can hope to do in an instance like the present 

 one is, to select those more conspicuous forms which stand forth as it were from 

 the rest, and constitute local foci from which subsidiary rays would seem in a 

 measure to branch out. 



Regarding the distribution of the Ptiims under consideration, it would appear 

 to be rarer on the large than on the small islands of the group. Thus, in Madeira 

 proper it is, so far as I have hitherto remarked, decidedly scarce. In Porto Santo 

 it is far less so, occurring from intermediate altitudes to the very siunmits of the 

 movmtains, — where I have taken it, during the early spring, from amongst the 

 dense Hchen (particidarly Ramaliua scopu'lorum and Evernia prunastri) which 

 gathers around the crevices and inequalities of the weather-beaten peaks ; and I 



