104 



interfered with, it usually happens that the others are 

 more or less involved with them : but we may offer a 

 few desultory remarks, which will tend to show that 

 disturbing agents are apt to mar them both individually 

 and as a whole, and not only so, but to affect them in 

 a permanent manner (as indeed has been already inti- 

 mated), according as similar combinations of them are, 

 from local causes (as it were), selected, to be acted 

 upon. 



I have stated in the last section of the preceding 

 chapter that insect stature is eminently beneath the 

 control of contingences from without; adducing, amongst 

 other examples, in support of this, the Madeiran Ptinus 

 albopictus, a species which, whilst it averages more 

 than a line in length on the central island of the group, 

 is reduced to less than half that bulk on a small and 

 weather-beaten rock (the llheo Chao) at a distance from 

 it. Judging indeed from many hundred specimens of 

 the Plini which I have submitted to a close comparison, 

 ' ' the most constant of their characters would seem to be 

 outline and sculpture, whilst size and colour are appa- 

 rently the least to be depended upon : so that trifling 

 differences may be of specific indication in the former 

 case, where in the latter much larger ones are worth- 

 less*." I have in fact generally noticed, that size and 

 colour are more peculiarly liable to be affected together. 

 This, however, is nothing more than what we should 

 anticipate, since the same causes which have stunted the 



* Insecta Maderensia, pp. 260, 261. 



