INTRODUCTION. 5 



species ami genera on mountains in the Tropical and Temperate zones, 

 and in high latitudes of the old world, is an erroneous one. The 

 difterent state of things in the new world is probably due to the 

 existence of some obstacle to free migration, as far as regards Insects, 

 between north and south, both during and since the Glacial epoch. 

 The problem, like most others relating to Geographical Distribution, 

 is a complicated one ; but there are one or two considerations, likely to 

 ho, overlooked, Avhich may tend to its solution. One is the great alti- 

 tude at which the vigorous denizens of the teeming tropical lowlands 

 flourish on the slopes of the Andes. INIr. Whymper found, for example, 

 species of many of the genera of Longicorn Coleoptera characteristic of 

 the lowland forests at altitudes of 9000 and 10,000 feet, and Kirsch 

 has recorded numerous species of Lainjii/rida', Lycuhv, and other families 

 lielonging equally to Tropical American forest genera, as met with by 

 Reiss and Stiibel in Colombia and Ecuador at 12,000 feet. In Ecuador 

 all the w^arm moisture brought by the Eastern trade-winds is not inter- 

 cepted even now by the wall of the Andes, and wherever that falls, in 

 the depressions, conditions of climate and vegetation will he created 

 suitable to these encroaching Tropical forms. If we add to this the 

 barrenness and generally unfavourable conditions of the zone above 

 those altitudes, there can be little wonder that Temperate forms have 

 not freely passed along the Andes. Another consideration is that there 

 may have been a breach of continuity of the land in Glacial times, at 

 the Isthmus of Panama, sufficient to })revent free migration. It may, 

 further, be legitimate to speculate on the possibility of the Andes being 

 lower in the Tropical zone during the Glacial epoch. A few hundred 

 feet lower than the present altitude, combined with the copious warm 

 rains which must have accompanied the age of ice, would present con- 

 ditions undoubtedly favourable to the spread of Tropical forms over 

 the whole area, which would successfully resist the invasion of higli 

 northern or southern species. The main principle in distribution, how- 

 ever, is that forms sooner or later, and in proportion to their intrinsic 

 and extrinsic facilities of dissemination, will find their way all over the 

 world to wherever the conditions inorganic and orjianic are favourable 



