172 On the Geographical Distribution of Insects, 



;ue singularly augmented in number as we approach more 

 southern countries. This circumstance, moreover, is in har- 

 mony with the increase of vegetables m the same direction, and 

 the enormous mass of detritus which they deposit on the ground 

 in intertropical regions, detritus which is not of such a nature 

 as to disappear rapidly, like dead animal matter, under the ac- 

 tion of the solar rays. 



On i-eviewing all the species of all the orders existing in 

 iinglaud, Kirby and Speuce estimate the Phyto-saprophaga of 

 that country to be to the Phyto-thalerophaga as 1 : 9. Push- 

 ing their calculations further, these skilful entomologists con- 

 ceive that, among the latter, the fungivorous species are about 

 3u, iind the gi'anivorous ^V ; and that, among the Phyto- 

 saprophaga, the half ai-e lignivorous, and ^ coprophagous. 

 These proportions nearly correspond to those obtained by 

 M. Lacordaii'e for the insects of France. 



On the extent of the habitation of Species, Genera, and Fa- 

 iuilieit.—MiQV having studied the relative proportions of the 

 various entomological groups, it is natural to inquire as to the 

 space which each of them occupies on the globe, for all have 

 their limits, beyond which they never extend. The space 

 comprised vv'ithin these limits constitutes what is called theh 

 area. This area is continuous when all the countries com- 

 posing it are contiguous to each other, and form an unin- 

 terrupted whole ; disjoined, when the contrary is the case. 

 These terms, however, must not be taken too literally ; and 

 when two countries are separated only by a small interval, as 

 the most part of islands are from the continent to which 

 they belong, they ought to be considered as united. Their 

 vicinity permits species easily to pass from one to the other, 

 and it is \evy probable that they would be found in the in- , 

 termediate places, if any such existed. Thus, Madagascar 

 could not, with any propriety, be separated from the coast of 

 Africa, opposite to which it lies ; and it is found that a great 

 number of species are common to both. 



Mountains are very often the cause of the disjunction of 

 areas, aiid may be regarded as islands in the middle of con- 

 tinents. 



