The Scottish Naturalist. 343 



sects may, with favourable winds, be carried across the Pentland 

 Firth. In this way additions may not only be made to the list 

 of species, but — what would tend greatly to prevent the develop- 

 ment of insular varieties — individuals of already existent species 

 would introduce fresh blood. 



The Outer Hebrides, on the other hand, are more remote, 

 and have been longer isolated. Into them, also, it is not im- 

 possible that occasionally new colonists are introduced, but 

 much more rarely than into the Orkneys. Hence there have 

 been time and opportunities for the development of local races, 

 and, as Mr Jenner Weir remarks, the environment has been very 

 favourable for such development. 



In the "battle of life" insects of the order Lepidoptera rarely 

 depend upon weapons of offence or defence for protection from 

 their numerous enemies, but by the assimilation of their colour 

 or shape to those of neighbouring objects. Hence the green 

 ground-colour and lines, bands, and spots of many caterpillars ; 

 the mottled grey., black, and white of the wings of moths that 

 rest on tree-trunks ; the mimicry of bits of stick and withered 

 leaves that others present ; and the numerous other contrivances 

 of protective resemblance. No one of course believes that 

 insects consciously assume these colours and forms. Such pro- 

 tective resemblances are gradually brought about by the law of 

 natural selection, whereby the species or varieties best fitted for 

 the locality will survive, and those less fitted perish. Now a 

 form that in one locality will be the fittest will in another be less 

 adapted for the surroundings, and unless natural selection inter- 

 venes, will probably soon perish. When natural selection inter- 

 venes its action is this. All species have a greater or less 

 tendency to variation. Those individuals which vary — be it 

 ever so little — in the direction of a greater protective resem- 

 blance, will survive and propagate the species when other less 

 favoured individuals fall a prey to enemies. In the next gen- 

 eration this resemblance will be more intensified in some indi- 

 viduals than in others, and the same weeding process will go on. 

 This will be repeated generation after generation, till in course 

 of time the whole race has acquired the protective form or 

 colour, and any relapses to the ancestral type (which will pro- 

 bably occasionally occur) will be more readily discovered (being 

 less protected) by enemies, and hence not have an opportunity 

 of reproducing their like. The insects of the Outer Hebrides 

 show many examples of this law of nature. As an instance we 



