GEOGRAPHICAL DISTRIBUTION 251 



Climate 



Climate is the expression of a combination of physical factors, 

 and is of paramount importance in connection with the survival 

 of all forms of life. The effects of the more important of the 

 individual factors which collectively make up a climate have 

 already been referred to, and their combined influence now remains 

 for consideration. Climate may be defined as the complex of 

 meteorological factors prevailing throughout the year for a given 

 area, while the incidence of these same factors at some particular 

 point of time constitutes weather. Climate in the broader sense 

 influences the geographical and ecological distribution of species, 

 while weather influences their local prevalence by seasons : the 

 one operates in space, the other in time. 



Geographical Distribution. It is well known that the vast 

 majority of insects are more or less restricted to definite areas. 

 Without taking into account past geological changes, the factor 

 governing such distribution at the present day is very largely 

 climatic. On the other hand, topographical barriers, such as 

 mountain ranges, deserts and oceans, play an extremely important 

 part. Their influence, it is true, is partly in relation to the climates 

 they engender, but, apart from such, they remain as obstacles 

 which few species can overcome. The tsetse fly, for example, 

 is unable to survive the ecological conditions presented by the 

 Sahara desert, and consequently does not penetrate north of that 

 vast area. Similarly, it has not spread into Asia, where the 

 Arabian desert presents a more formidable barrier than the 

 relatively narrow strip of water forming the Red Sea. The Atlantic 

 Ocean effectually precludes the spread of most elements of the 

 western European fauna into North America, and it has only been 

 due to the relatively recent facilities, afforded by maritime 

 communications, that certain species of insects have become 

 accidentally transported across that barrier. The latter event is 

 one of outstanding importance, since it demonstrates that once 

 topographical barriers are overcome, some species of insects find 

 other climatic areas at least as favourable for their colonisation as 

 their lands of origin. The significance of this fact is becoming 

 increasingly appreciated, and is witnessed in the rigidity with 



