210 THE ENTOMOLOGIST. 



widens into North America, and has a large arctic expanse. 

 Africa differs widely in aspect from America, it being limited, 

 with the exception of the Isle Tristan da Cnnha, the Isle of 

 Desolation, and a few others, to the middle part of the 

 earth : its north division, as regards the insect Fauna, 

 includes Arabia and the countries bordering on the Medi- 

 terranean, and occasionally a few of its native insects appear 

 on the coast of England. The species in St. Helena and in 

 Madeira would, perhaps, he found to have greater resem- 

 blance to those of Africa, when the insects in the mountains 

 of West Africa are more known. 



The tropical regions of Asia are not compact like those of 

 Africa and of America, but are composed of numerous 

 islands ; and the southern part of this division is chiefly 

 formed by Australia and by New Zealand. Its northern 

 division includes more land than any one of the five before 

 mentioned, and is a wide field for the investigation of the 

 distribution and range of insect species thereon. The 

 migration of insects from east to west in this division will 

 account for the absence of many species in western districts, 

 owing to the difficulties they experience in surmounting 

 intervening obstacles before they can find suitable spots for 

 effecting stations. It will also account for the occurrence of 

 species in widely-separated localities, and for their absence 

 in the intervening space, where climate, or soil, or vegetation, 

 or cultivation, has not allowed them to make or to continue a 

 settlement. 



It has been said that during the gradual decrease of the 

 glacial epoch, its retreat was followed from south to north by 

 insect species, and that these in many cases wholly left the 

 south of continental Asia, to which they were formerly 

 limited. In this progress they would be hindered by the 

 Himalayas and by other alps, and by the vast plains of 

 14,000 to 16,000 feet elevation beyond, and would find less 

 obstruction in a passage through China and Mantchouria to 

 Saghalian-oula or Amurland, or the large region of the river 

 Sagh alien or Songari and of its tributaries. In this journey 

 some species would ascend and colonise mountains, and 

 thereby epitomise and anticipate the present arctic insect 

 race. The correspondencies between alpine and arctic 

 insects in Europe are well known : they have been less 



