Geographical Distribution 297 



parison of such facts as these suggests to us the 

 fortunes of the various species in the battle of life. 

 The widespread forms must be dominant and vigorous, 

 while those with a confined or discontinuous range 

 in the hill-country and the west must be the older 

 members of the fauna, driven out of the eastern 

 plain which they once held. The insects with a 

 south-eastern distribution represent, on the other 

 hand, the newest immigrants into our islands which 

 have not been able to force their way far to the 

 north and west. 



Such distributional facts as our own islands furnish 

 on a small scale can be studied on a grand scale 

 in the world at large. Some vigorous and dominant 

 species — such as the "Painted Lady "Butterfly Pyrameis 

 cardui — are to be found in all quarters of the globe, 

 while a vast number of insect-families are cosmo- 

 politan ; the Nymphalidfe, for instance, to which 

 this butterfly belongs, swarm in the tropics and are 

 represented at the far north of Greenland. Many 

 groups of insects, such as the Papilionidae among 

 butterflies and the Cicindelid^e among beetles, are 

 most abundant in tropical countries and are found 

 to decrease in numbers and importance as the observer 

 journeys north or south from the equator. Others, 

 however, as the Sawflies (Tenthredinidx) and the 

 Rove-beetles (Staphylinidae) are specially character- 

 istic of the great northern or Holarctic Region, 

 dying out towards the south. 



It is interesting to find that the regions into which 

 the earth's surface has been divided, by students of 

 vertebrate zoology (172, I73)> serve fairly well to 

 indicate the distribution of insects. The following 

 seven regions are perhaps the most satisfactory in the 

 present state of our knowledge : — 



