77 



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 obvi- 

 ously 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 Ptiliada \ "* 



I stated above, that, although this diminution of 

 stature is a very general accompaniment of isolation, 

 amongst insects which have been long cut off from the 

 rest of their kind, there is no rule without an exception 

 to it ; and that, therefore, we must not always anticipate 

 the result which has been described. We should re- 

 member that immense periods of time are apparently 

 necessary before any perceptible change can come over 

 creatures from the stoppage of their migratory progress, 

 and the unnatural in-breeding of their several tribes ; so 

 * Insecta Maderensia, pp. 268, 269. 



