280 GEOGRAPHICAL AKD GEOLOGICAL DISTRIBUTION. 



erly points reached by man they appear to be still remarkably 

 abundant. The officers of the British North Pole Expedition, 

 under command of Sir George Nares, brought home a surpris- 

 ingly rich fauna from the region (Grinnell Land) lying between 

 the seventy-eighth and eighty-third parallels of latitude, com- 

 prising no less than forty-five species of true insects and sixteen 

 arachnids, the former distributed as follows : Hyrnenoptera, five 

 species (two humble-bees) ; Coleoptera, one ; Lepidoptera, thirteen ; 

 Diptera, fifteen; Hemiptera, one; Mallophaga, seven; and Collern- 

 bola, three. 102 Among the Lepidoptera are a number of forms 

 belonging to genera common in the temperate zones, such as Co- 

 lias, Argynnis, Lycsena, &c., which appear the more remarkable, 

 seeing that the species of this order are more limited in Green- 

 land (with an insect fauna numbering eighty species), and that no 

 forms are met with either in Iceland or Spitzbergen, although up- 

 wards of three hundred species of insects are represented in the 

 former. On the other hand, M. Bonpland observed butterflies on 

 the slopes of Chirnborazo at an elevation of 16,626 feet, or but 

 1,600 feet below the highest level (18,225 feet) reached by insects 

 (Diptera), as observed by Hurnboldt, on the same mountain. 



Insect life flourishes also to a certain extent in waters of high 

 temperature (hot springs) ; and even on the free surface of the ocean, 

 most distantly removed from land, the officers of the ' ' Challenger " 

 circumnavigating expedition everywhere obtained one or more spe- 

 cies of Halobates, a member of the Hemiptera, which is stated to 

 live entirely at sea, and to carry its eggs about with it attached to 

 its body. 



The distribution of insects is determined largely by climatic and 

 general physiographical conditions, and also to a great extent by 

 the nature of the food-supply, many forms, as has already been 

 seen, being dependent for their sustenance (whether in the larval 

 or mature state) upon the development of a particular vegetable 

 product. Indeed, localisation or restriction appears to be more 

 frequently brought about as the result of changes in the character 

 of the vegetation than as a condition depending upon the interpo- 

 sition of physical barriers which, through their powers of flight, 

 the animals of this class are in great measure able to overcome. 

 The effect of climate is, however, well marked, and is seen in the 

 general restriction of numerous forms to particular climatic zones. 



