55 



able will it be for the development of insect life. Mr. 

 Darwin has noticed this fact in the Galapagos (which he 

 remarks are only equalled by Tierra del Fuego, in bar- 

 renness), on Keeling Island (in the Indian Ocean), where 

 he succeeded in detecting but thirteen species, in St. 

 Helena, and at Ascension ; and I have added fresh 

 evidence to the same in the various portions of the Ma- 

 deiran Group*. It is however to geological causes that 

 we must mainly look for the explanation of this pheno- 

 menon ; and, therefore, since I propose to examine that 

 branch of our subject in a future chapter of this treatise, 

 we will not discuss it now. It will also be better per- 

 haps to defer for the present the general question of 

 se\f- diffusion, which, at the opening of this section, we 

 proposed to consider, along with that of insect product- 

 iveness (as dependent on other local influences, besides 

 climatal ones), it being scarcely possible to render the 

 problem of dispersion in any degree intelligible without 

 calling in geology to our aid. 



* Considering that I have already detected more than one thou- 

 sand species in those islands, it may perhaps be questioned whether 

 the same truth is to be gathered from the result of my Madeiran 

 researches. I would wish it therefore to be understood, first, that 

 my statement refers to that group as contrasted with countries in a 

 similar latitude ; and, secondly, that its real fauna is alone taken 

 into account, the host of introductions from more northern regions, 

 a large proportion of which have probably taken place within a very 

 recent period (as may be fairly presumed from the knowledge that 

 fresh arrivals, an almost necessary consequence of the importation 

 of plants, are occurring nearly every season), having been dismissed 

 from our present inquiry. 



