89 



since they go very materially to prove that the effects of 

 isolation on external insect form are even more import- 

 ant, if possible, than those of latitude. That this is 

 the case in the present instance, appears clear from facts 

 so minute as these. For, out of the many specimens 

 which have come under my observation from various 

 countries of Europe, if there is one point more constant 

 than another in this otherwise variable species, it is, I 

 believe, under all circumstances, its immaculate pro- 

 thorax. Now, whilst this (we may almost say essential) 

 character obtains in Porto Santo, in Madeira it does 

 not hold good : the prothorax there is invariably infus- 

 cate in the centre; and on a small adjacent rock (the 

 Ilheo de Fora) it is entirely dark. Nor let anyone sup- 

 pose that details apparently so trivial are beneath our 

 notice, or the mere result of chance, since it is by the 

 observation of such-like points, and by marking their 

 development according to the circumstances of the 

 several localities in which they obtain, that we are alone 

 able to appreciate their importance, and so to form, in a 

 wider and geographical sense, a correct estimate of their 

 value*." The Olisthopus Maderensis, Woll., is much 

 paler, larger, and more opake, on the Dezerta Grande 

 than it is in Madeira proper. So great indeed is the 

 change which it has undergone through a long isolation 

 on that rock, " that, had the case been a solitary one, I 

 should not have hesitated in regarding the specimens 

 obtained from thence as specifically distinct; neverthe- 

 * Insecta Maderensia, p. 6. 



